Xenomorph (Earth-5875)

"That which you know as xenomorphs are your salvation through destruction."

- Gravemind

The xenomorphs, designated as LF.Xx.3273 by the forerunners (Latin Linguafoeda acheronsis meaning "foul tongue from Acheron"), and referred to as the parasite, the serpents, and the infection by the Covenant species, are a highly aggressive sapient endoparasitoid species that can quickly reproduce and grow through the infection or impregnation of other species with sufficient cognitive capabilities. With origins shrouded in mystery and uncertainty, xenomorphs reproduce by spreading its infection towards other organisms, hijacking their bodies and impregnating them with a Chestburster, which in turns is converted into one of the species' many specialized forms. By impregnating other species, xenomorphs become increasingly more intelligent, and because of their frighteningly rapid growth rate, they are effectively unstoppable.

All xenomorphs share one single consciousness, which increases with each host it impregnates, eventually forming the ultimate xenomorph shared mind known as a Gravemind: a self-aware and incredibly intelligent entity which commands all other xenomorphs during outbreaks. Because of their group-like consciousness, the xenomorphs are better described as a single, trans-sapient macro-organism instead of individuals. When a Gravemind is not present, xenomorphs are only basically coordinated through the Feral Stage, where they seek to build more and more biomass through hives, leading to the creation of Proto-Keymind and Keyminds in the Coordinated Stage. Xenomorph's collective consciousness can only be described by humans as likening to a socialistic utopia, with each component of the species working to ensure their survival.

Origins
The exact origins of the xenomorphs is shrouded in mystery and widely disputed, with numerous theories, more than often conflicting, being proposed as known by the very few accounts of the Forerunner-Xenomorph War and other forerunner archives. It is believed that the xenomorphs, much like the huragok, are not actually natural, but rather, an artificially engineering species meant to act as a biological weapon by another technologically advanced species, theorized to be the forerunners themselves and even the more ancient and mysterious precursors. This theory relates to the mysterious chemical A0-3959X.91 and LF.Xx.3273, the xenomorph super cells. Evidence of their unnatural origin tend to be supported by the xenomorphs' biomechanical physiology and its perceived "perfection" by many individuals. Extragalactic in nature, other theories suggest the xenomorphs came from the Andromeda galaxy.

Forerunner-Xenomorph War
"It fed on intelligent life and in doing so, became ever more intelligent itself. The xenomorphs were unique; the perfect organism. It used the forerunner's very strength against them."

- - Cortana on the Forerunner-Xenomorph War The xenomorphs emerged in the Milky Way galaxy following the seeming first contact between them and the forerunners on the barren planet G 617 g. The forerunners underestimated the threat of the endoparasitic lifeform, and focused on controlling the "disease" rather than commencing a major war. However, by the time the outbreak of the xenomorphs started to spread through the galaxy, the forerunners realized the mortal danger of their new foes. While initially thought to be unhinged animals, the forerunners discovered the xenomorphs to be sapient, as they amassed enough biomass and harvested enough hosts to create Proto-Keyminds, Keyminds, and eventually, Graveminds, which was the embodiment of their entire collective intelligence. They could coordinate swarms of xenomorphs to attack the vast forerunner fleets using all of their power, including telepathic power.

Xenomorphs would assemble in huge numbers against civilian ships so they could enter planets by trespassing their orbital fleets, and then, once arriving on the planet, harvest and impregnate all the population. The xenomorphs on these planets numbered in the billions, far surpassing the number of forerunner Warrior-Servants, which themselves became potential xenomorph hosts. These swarm tactics were observed on both LP 656-38 e, DM-3-1123b, and LV-426. The forerunners resorted to orbitally bombard the infested worlds, which, while effective, led to the death of billions of forerunners that could not be evacuated. This reality was seen through most of the Ecumene's victories against the xenomorphs. When Warrior-Servants were overwhelmed, the major cities in colonies committed mass self-suicide with massive weapons of destruction to stop the xenomorphs from getting them.

This desperation led the forerunners to realize a conventional warfare was ineffective against the xenomorphs, which grew with each passing day, so they began to rely on crueler strategies. The forerunners used their robotic Sentinels to contain the xenomorphs, while they used their massive warships to send stars into supernovae, taking with them entire planets and systems filled with xenomorphs to prevent the parasite from spreading. The Ecumene would suffer their biggest blow when Mendicant Bias, a Contender, succumbed to the Gravemind's Logic Plague, using his resources against his makers.

Final decade
As the xenomorphs continued to grow exponentially, they spread through system to system merely with the intent on reproducing and spreading as opposed to engage with forerunner ships, leading to the infestation of several core worlds. The Graveminds became planet-sized, and their intelligence easily surpassed that of forerunner Artificial intelligence. The Logic Plague continued to indoctrinate other Contenders and Metarchs. This proved to be an effective blow on the forerunners, as AI and automation was their mean means of combat, their society having lived without any wars for a millennia. This ability led the xenomorphs to utilize their biological assets with forerunner machinery and even access slipspace.

After trying to execute over a thousand plans to contain and extinguish the xenomorphs, the forerunners failed in all of them, until they decided to use a system of seven ring-shaped weapons, known as the Halo Array, which would purge the galaxy of all life, including the xenomorphs and their own. As the xenomorphs were endoparasites, their survival and reproduction depended on hosts, and so the forerunners reasoned that once every potential lifeform was gone, the xenomorphs would be unable to spread. The decision was met with many fights, but the Ecumene Council eventually activated the Halos in 97,445 BCE.

The pulse of the seven rings purged all life in the Milky Way. The only surviving xenomorphs were those kept in highly protected research and containment facilities across the galaxy, from the Halos, to shield worlds, to the Threshold Mine and the Derelict vessel on Acheron. Only very few sapient and sentient species of the galaxy survived, having been rescued by the forerunner Lifeworkers from planets not yet claimed by the xenomorphs. They were indexed in Installation 00 and safely returned to their homeplanets in Keyship, which later departed from the planets once the species had been reintroduced.

Massacre on the Nostromo
Thousands of years after the end of the Forerunner-Xenomorph War, on June 8th, 2528, the human civilian freighter Nostromo returned from an operation at Thedus when it received a mysterious communication coming from the natural satellite Acheron, in the Zeta Reticuli system. Upon landing, crew members Arthur Dallas, Thomas Kane and Joan Lambert stumbled upon a derelict ship stranded on the planet. After an inspection, Kane discovered a room that was previously sealed, and when he entered, he was attacked by a Facehugger. While Ellen Ripley insisted that the crew followed the security protocols, the science officer Ash, secretly a synthetic sleeper agent sent by the Office of Naval Intelligence, planned to take the creature, as ONI knew about the nature of the signal all along.

The act culminated into the death of all but one of the crew members, Ripley, who was able to combat the resulting fully-grown xenomorph that took out the crew one by one. During her survival, she also came across a Proto-Keymind, which she was successfully able to kill. However, her escape left her stranded in the shuttle Narcissus, which had no Shaw-Fujikawa Translight Engine, thereby forcing her to travel at superluminal speeds in the hopes of being encountered.

Outbreak of Etran Harborage
On February 23rd, 2531, the human ship UNSC Spirit of Fire performed a slipspace jump from Arcadia after following a signal supplied to them by Professor Ellen Anders, who was captured by the Covenant, leading them to the shield world known as Etran Harborage, whose containment measures had failed and allowed the megastructure to become filled with xenomorphs. UNSC Marines led by Sergeant John Forge were deployed to the planet after seeing unknown hostiles, where they were attacked by multiple xenomorphs, leaving Forge as the only survivor. Forge then rallied more forces to create a firebase, discovering a massive Proto-Keymind attached to xenomorph hives. The Proto-Keymind had copied Anders' signal to lure the humans and spreading beyond the planet. SPARTAN-II Red Team, alongside the UNSC Marines, were able to destroy the Proto-Keymind.

To prevent the Covenant from acquiring a fleet of forerunner Sojourner-class dreadnoughts, the crew of the Spirit of Fire decided to use the reactor of their own Shaw-Fujikawa Translight Engine to induce the artificial star of Etran Harborage in a supernova state. The plan was successful, at the cost of Forge's life, and the explosion killed all xenomorphs and Covenant forces that remained in the planet, alongside the dreadnought fleet. However, as a result of sacrificing their engine, the Spirit of Fire was lost in space at superluminal speeds.

Harvesting of Hadley
On June 28th, 2536, Russ and Anne Jorden, two spacers and inhabitants of Hadley, in the colony Paraíso, brought a cargo from Acheron as ordered by the Office of Naval Intelligence, following the rediscovery of Ellen Ripley in deep space earlier the same year. Once they arrived on the planet with the cargo on July 10th, 2536, Russ was attacked and impregnated by a Facehugger, leaving to a full-scale outbreak on Paraíso. A hive and a Proto-Keymind were soon born at Hadley's Atmosphere Processing Plant, which eventually evolved into a Keymind itself. The xenomorphs killed, harvested and impregnated all inhabitants of the colony, with the exception of Rebecca "Newt" Jorden.

The UNSC Sulaco of the UNSC Navy, along with a massive unit of UNSC Marines was dispatched at the colony on July 27th, 2536 to investigate the sudden loss of contact, accompanied by Ripley herself. This led to numerous battles and encounters with the xenomorphs that led to the deaths of nearly the entire Marine unit and the Sulaco crew. Eventually, Ripley, with the help of the synthetic Bishop and Dwayne Hicks, was able to destroy the Atmosphere Processing Plant and kill all xenomorphs present. The Keymind has secretly hidden in the Sulaco, but it was killed by Ripley, who ejected it into space.

Brief reappearance
On January 1th, 2537, exactly six years after the outbreak on Etran Harborage, a new outbreak started aboard the UNSC Spirit of Fire, after a single Facehugger that stood hidden aboard the ship attacked and impregnated engineer Violeta Maldini. This led to the creation of other xenomorphs that threatened to break a quarantine established by the ship's First Mate, the synthetic Serina, who then awoke Professor Ellen Anders and SPARTAN-II Jerome-092. The duo was able to destroy all xenomorphs aboard the ship and ensure the survival of the Spirit of Fire's crew.

Revival on Terra Nova
On November 7th, 2547, the Office of Naval Intelligence closed businesses of the colony Terra Nova to pan-corporate enterprises, reneging numerous co-development agreements, following the discovery of forerunner ruins on the planet. This later escalated in a major ONI effort to cover the purpose of a major facility they had built amidst the forerunner ruins, conducting research on the forerunners and xenomorph specimens it managed to retrieve from Paraíso thanks to Carter Burker. These xenomorphs were enslaved and used as test subjects to form a Keymind. The Keymind was eventually able to escape with the help of a lone and intelligent xenomorph, and quickly settled a hive within Freyja's Staten Refinery. The catastrophic outbreak led to the slaughter of the population of the settlement of Freyja, except for its synthetic administrator Katya, who was able to hide and warn the UNSC Marlow, the closest ship to the planet, about the mysterious alien attackers. Believing the aliens to be an unidentified Covenant species, the UNSC Marine Corps then deployed its 1st Battalion, 3rd Regiment to Terra Nova, to investigate. However, as soon as the Marlow arrived in the Iota Draconis system, it was shot down by a hidden Covenant CCS-class battlecruiser, led by a group of Silent Shadow sangheili, who were in search of a Special Operations team who had gone missing on the human world.

After a three-sided battle between the UNSC, the Covenant, and the xenomorphs, Terra Nova's settlements, laboratories, and forerunner ruins were destroyed by the First Blade officer Dahr 'Voliree with a stolen HAVOK tactical nuke in order to prevent the xenomorphs from escaping from the planet and spreading across the galaxy. After its loss, the Office of Naval Intelligence covered the event as being one of the many attacks by the Covenant, and had the UNSC prohibit any transit or approach to the planet.

Isolation on Sevastopol Station
On December 10th, 2547, Henry Marlow, the captain of the salvage vessel Anesidora arrived at Seegson's Sevastopol Station, requesting medical attention to his wife, Catherine Foster, who had been impregnated by a Facehugger. After Dr. Kabira Lingard, head of Sevastopol Station's medical staff, attempted to remove the creature, she was blackmailed by Ben Ransome of Chief Operations to tell him about the alien. Ransome then convinced Marshal William Waits of the CMB to skip the routine quarantine after finding the flight recorder of the Nostromo. This led to a Chesbuster hatching from Foster, who escaped through the station and grew, killing every human and Working Joe through its way.

This was only averted with the arrival of Amanda Ripley-McClaren, who worked alongside Marshal Waits to trap the massive xenomorph in a shuttle to separate it from the station and send it towards the gas giant Asimov, killing it.

Survival on Fiorina 161
On July 19th, 2552, Ellen Ripley and the other survivors of the outbreak on Paraíso were jettisoned from the UNSC Sulaco in a Class-3 Bumblebee, following an attack by a Facehugger that led to an electrical fire in the cryosleep compartments, crash-landing on Fiorina "Fury" 161, which led to the deaths of both Newt and Hicks, leading Ripley and a heavily damaged Bishop as the only survivors. Unknown to them, a Facehugger had emerged from the Bumblebee and impregnated Spike, a guard dog at the Fiorina 161 Class C Work Correctional Unit. During the funeral for Hicks and Newt, xenomorph emerged from Spike and matured, starting to systematically kill the facility's inmates and leading to mass hysteria. Ripley organized the prisoners to defeat the creature, but a plan involving explosives to kill the creature in the ventilation shafts backfired and killed most inmates.

An Office of Naval Intelligence team of combat synthetics, led by Michael Bishop, arrived to rescue Ripley from the planet after being contacted by Francis Aaron. Bishop attempted to convince Ripley to turn herself to ONI so they could extract the Keymind embryo that resided within her. As she did not trust Bishop, Ripley chose to sacrifice herself in order to kill the Keymind Chestbuster to keep ONI from getting their hands on it. She did so, throwing herself into one of the prison's furnaces.

Resurgence on Installation 04
On September 19th, 2552, humanity and the Covenant stumbled upon Halo Installation 04, after the synthetic executive officer Cortana of the UNSC Pillar of Autumn performed a slipspace jump from the coordinates she acquired from an artifact found at Sigma Octanus IV in order to escape from the Fleet of Particular Justice at the Fall of Reach. This lured both forces to Installation 04, and in turn, encounter the xenomorphs that were stored on the ring. After the Autumn crash-landed on the ring, a battle soon ensued.

The Covenant crew of the Truth and Reconciliation amassed their forces on the ring, stumbling upon an underground xenomorph containment facility in the ring's swamps. The Covenant used the area as a fortification against the UNSC, oblivious of the danger they faced. Through a blunder of the Covenant's part, the containment was open, and soon Covenant warriors went check the Ovomorphs, which hatched Facehuggers that attacked and impregnated them, leading to massive outbreak. As the Covenant had knowledge over the xenomorphs, they locked the facility and evacuated the surviving warriors. Captain Jacob Keyes and his team of UNSC Marines were unaware of the peril of the facility, which they continued to move on deeper, leading to the release of the xenomorphs that attacked and harvested them. John-117 was dropped to his location, only to discover Keyes had disappeared and the entire Marine unit either slaughtered or impregnated by the xenomorphs after watching the helmet recorder of Private Wallace Jenkins.

The ring's Sentinels were unable to contain the xenomorphs, leading to heavy casualties for both the UNSC and the Covenant, with the xenomorphs started to occupy areas previously used by the Covenant as hives. Against all odds, John-117 and Cortana were able to detonate the Autumn's engine reactors, before escaping the ring in a GA-TL1 Longsword. The thermonuclear explosion led to the destruction of the Halo ring and stopping it from being activated by the Monitor 343 Guilty Spark, and killing all xenomorphs present on the installation.

Operation RED CLOUD
Following the destruction of Installation, the human penal ship Mona Lisa arrived in the Soell system on October 25th, 2552. Headed by Office of Naval Intelligence agents, the agency sought to acquire new xenomorph specimens for study and potential weaponization following the loss of Ellen Ripley on Fiorina 161. Testing the parasite on both human and sangheili prisoners, the operation soon went awry when the xenomorphs broke quarantine and infested the ship, harvesting most of the crew and inmates, safe from Major John Smith, Patrick Himmler, and "Henry", eventually gathering enough biomass to form a Proto-Keymind.

The loss of contact was eventually investigated by Sergeant Zhao Heng Lopez and her squad of UNSC Marines from the UNSC Red Horse. After the loss of practically all of her squad, Lopez and Henry fought against each other for the chance of escaping the ship in a single Class-3 Bumblebee, whereupon the Mona Lisa was destroyed by the Red Horse with a Shiva-class nuclear missile.

Unleashed on the Threshold Mine
On October 20th, 2552, a small Covenant Special Operations team led by Special Operations Commander Rtas 'Vadumee and Arbiter Thel 'Vadamee were sent by the Hierarchs to kill the heretic leader Sesa 'Refumee and his faction, the Light of Truth, which claimed the Great Journey was a lie. Upon arriving, the heretics released the xenomorphs stored in the station to kill the Special Operations team.

Their plan backfired when the xenomorphs were also able to get their own members, impregnating and killing many of their forces, and failing to kill the Arbiter. Breaking from the station's left arm, they quickly spread to the mine's central core, overwhelming the heretics and the Sentinels there present. The Arbiter was able to kill all xenomorphs and Light of Truth members in the station when he severed the sustaining cable that attached the mine to a support structure in Threshold's orbit, sending the station to plummet at the planet's core.

Battle of Installation 05
The xenomorphs were in a never-ending battle against the Sentinels of Installation 05 following their escape from a securing facility duo to the irresponsibility of its Monitor, 2401 Penitent Tangent. After the infestation was considered critical, a massive Quarantine Zone and Sentinel Wall was established so that the xenomorphs could never reach the installation's Library. The outbreak was so massive, the xenomorphs were able to create the only known Gravemind of the current era, who established a massive hive deep within the ring's subterranean areas. After the Covenant and the UNSC arrived at Installation 05 on November 5th, 2552, the xenomorphs quickly turned their attention to their new potential hosts, which they successfully harvested to amass even more power and thus gain victory over the Sentinels.

Despite their efforts, the xenomorphs were unable to stop the UNSC and the Covenant to further access the ring, and Commander Miranda Keyes manned the UNSC In Amber Clad over the Quarantine Zone's Enforcer constructs, at the same time where Covenant forces led by the Arbiter and Rtas 'Vadumee used two anti-gravity gondolas to reach the Activation Index. After a UNSC defeat, the xenomorphs captured the In Amber Clad and used it to breach the Covenant's capital city, High Charity.

Attack on High Charity
The Gravemind was able to capture John-117, Cortana and Arbiter Thel 'Vadamee, who attempted to convince 'Vadamee against his own religious beliefs, warning to not activate the ring. The Gravemind then sent John and Cortana to search for the index in High Charity, while it sent the Arbiter to the Control Room of Installation 05. In fact, the Gravemind had merely used both of them, using the presence of John-117 and Cortana, alongside the Great Schism, to invade High Charity through the In Amber Clad and take over the entire city with its xenomorphs, supported by waves of xenomorph-controlled D77-TC Pelicans that released more and more xenomorph forms.

Both the Covenant's warriors and civilians were both ill-equipped and distracted with the Schism to properly defend themselves, and soon High Charity fell prey to the xenomorphs, including the Prophet of Mercy, who was attacked and impregnated by a Facehugger. Soon enough, the city had become a proper massive xenomorph hive, and the Gravemind would constantly rebut the words of comfort spoken by the Prophet of Truth, which he delivered to the population of High Charity while he himself escaped it unharmed. John-117 and Cortana were eventually able to reach Truth's Dreadnought, which would lead him to Earth. Cortana stood behind to detonate the In Amber Clad and take High Charity and the Halo with it in case Tartarus activated the ring, and was eventually taken by the Gravemind.

Invasion of Earth
After the Battle of Voi on November 18th, 2552 and the Prophet of Truth's activation of the Excession in the ruins of New Mombasa, the xenomorph-infested ship Indulgence of Conviction emerged from slipspace on Earth after they successfully penetrated the blockade established on High Charity by the then-recently formed Swords of Sanghelios, crash-landing on Voi and attempting to spread through the human homeworld. Commander Keyes and Fleet Admiral Sir Ernst Hood ordered John to detonate the Conviction's reactors and search for Cortana, which would lead to the destruction of Voi and the end of the outbreak. John and 'Vadam eventually received the aid of the Swords of Sanghelios' Fleet of Retribution, who sent numerous Special Operations sangheili to help them combat the xenomorphs. Together, they were able to reach the ship and kill all xenomorphs on their way. Upon reaching the ship, John found only a message from Cortana, with 343 Guilty Spark arriving to help him. With the xenomorph spreading, John, 'Vadam, the surviving sangheili and Spark boarded a Type-52 Phantom and went to the Shadow of Intent, while the Swords of Sanghelios glassed Voi to stop the xenomorphs from spreading, though at the cost of the devastation of a large portion of East Africa.

The Ark and defeat
When the Swords of Sanghelios and the UNSC Forward Unto Dawn went to the Ark through the Excession, they plotted to deactivate three shields that surrounded the Citadel, where Truth was hiding. Just as they deactivated the third shield, the xenomorph-infested High Charity emerged from slipspace at the Ark and crashed into the surface, releasing hundreds of xenomorphs onto the megastructure that infested Installation 04's replacement ring. When John and 'Vadam entered the Citadel, the Gravemind proposed a brief alliance so they could stop Truth from activating the Halo Array. After the Arbiter killed Truth, the Gravemind had no more serious opposition and ordered its xenomorphs to attack and kill the two.

After saving Cortana from the ruins of High Charity, John was able to kill all the xenomorphs on the Ark and Installation 04B after detonating the city's plasma reactors and activating the unfinished Halo with the Activation Index Cortana still possessed. Guilty Spark turned on the group after learning of the possible destruction of his ring, and killed Sergeant Avery Johnson, but in turn got destroyed by John. In its final moments of life, the Gravemind telepathically addressed John and the Arbiter, claiming that their victory would only slow down the xenomorphs, but not destroy them.

Aftermath
Despite the Gravemind's death and the eradication of its outbreak, the xenomorphs still continued to exist on other forerunner installations. Given the xenomorphs' extragalactic origin, it is possible that other hives and forces exist outside the galaxy, or, more disturbingly, that other distant galaxies may have fallen and fully taken over by xenomorphs. This fear was what drove the forerunners to keep xenomorphs alive in the Halo rings as opposed to eradicating them all, as they could return sooner than later, and a potential cure for their infection would be useful once they returned. The xenomorphs were declared defeated, but not without real worries.

The UNSC and the Office of Naval Intelligence kept the existence of the xenomorphs and the forerunners a secret from the UEG's population during and after the First Contact War. However, both the UNSC and their new allies, the Swords of Sanghelios, made a series of preparations for the possibility of the xenomorphs returning. The N7 was designed to become a first line of defense against the endoparasite, with multiple teams of SPARTAN-IVs and Swords of Sanghelios warriors being deployed on xenomorph contained areas on Installation 03. The SPARTANs aboard the UNSC Infinity run numerous TACTIM simulations which depict the potential xenomorph return.

Panick on the UNSC Auriga
Despite the Office of Naval Intelligence's previous failures, that did not stop them from wishing to acquire the xenomorphs, especially during the volatile post-war environment in which ONI plotted for counter measures and potential genocide on all the former Covenant species with biological weapons, which they saw in the xenomorphs as an untapped potential. In 2561, ONI started a secret program aboard the UNSC Auriga, where they sought to clone Ellen Ripley and the Keymind that was incubating within her. After seven failed attempts, which led to numerous monstrosities, they succeeded in creating the perfected Ripley 8 and Keymind clones. They had the smuggling human mercenary freighter Betty deliver them a series of convicts on the death row to the Auriga on September 17th, 2561, where they would be used as hosts to test the weaponization of the xenomorphs.

The xenomorphs were eventually able to escape, killing most of those aboard the ship despite General Martin Perez's best efforts to secure the Auriga's crew. Minutes before the outbreak, the ship's shipboard artificial intelligence Father had set a course for Earth for the Auriga to be docked, which would unleash the xenomorphs on the human homeworld and lead to a massive outbreak that could extinguish humanity. Alongside Ripley 8, the surviving members of the Betty, the surviving Marine Vincent Distephano, the ONI scientist Dr. Mason Wren, and the convict Larry Purvis fought against numerous xenomorphs on their way to reroute the ship to crash into Reach, which had been glassed years before by the Covenant and bore no life, to kill the xenomorphs.

Nightmare on the Ark
On August 3rd, 2565, the Banished and the crew of the UNSC Spirit of Fire became embroiled in a conflict over the control of the Ark. After the Banished lost the Enduring Conviction, the Banished's leader Atriox ordered the siblings Pavium and Voridus with salvaging any assets surrounding the ruins of High Charity, warning them not to enter as he was aware of the stories about the xenomorphs. Voridus simply ignored the request, believing the existence of the xenomorphs to be a mere propaganda of the Hierarchs to instill fear on the weak minded, and so he breached the containment shield that surrounded High Charity and entered the city.

This led to the release of the xenomorphs, with some Facehuggers having survived the firing of Installation 04B and the explosion of High Charity, which led to a bigger outbreak on the Ark. After Voridus, Pavium, and their jiralhanae warriors successfully killed the local Keymind, which was on the verge of evolving into a Gravemind, the xenomorphs were seemingly contained for a second time.

Biology
"You still don't understand what you're dealing with, do you? The perfect organism. Its structural perfection is matched only by its hostility. You admire it. I admire its purity. A survivor...unclouded by conscience, remorse, or delusions of morality."

- - Ash and Joan Lambert Xenomorphs are primarily solitary predators, but they are fully capable and willing to adopt swarm tactics when larger groups are formed. Even though they are highly voracious and can easily kill their enemies, xenomorphs prefer to let their victims alive in order to be used as incubators for Facehugger Chestbusters or spores. Victims can also be taken to hives and their biomass used to generate a Proto-Keymind. If their targets are deemed unsuitable for either infection or impregnation, however, xenomorphs do not hesitate into quickly killing their chosen host.

They are extremely resilient, being capable of surviving under extreme conditions which range from -75 to +53 degrees Celsius, as well as underwater, effectively being able to withstand the varied environments of all known inhabited planets through the Milky Way galaxy. They are, however, able to proliferate and fare better on moist and humid environments, with artificial cold environments being implemented on important areas through the Halo Array and other forerunner installations intended to hinder xenomorphs. Xenomorphs will transform the environment surrounding them to better suit their needs, altering the atmosphere of a planet and converting into more of their biomass.

Anatomy and physiology
Xenomorph height is varied between 8' 3" (251 cms) and 8' 8" (264 cms), while their weight average from 299 lbs (136 kgs) to 399 lbs (181 kgs). Xenomorphs will alter their physical appearance upon development or host, sporting different characteristics depending on the species they are infecting or impregnating; this ensures the adults are suited for the environment. This ability occurs at a cellular level under the xenomorph super cells, also known as "LF.Xx.3273" by the forerunners, and known as the DNA Reflex by xenoanthropologists. Most xenomorphs encountered in the modern era are gestated from the bodies of humans, sangheili, jiralhanae, unggoy, kig-yar, yanme'e and san'shyuum, and therefore they adopt bipedal stances, despite the existence of other variants that have been rarely found, from creatures and animals the xenomorphs have accumulated through Halo installations and unknown corners of the galaxy. Despite the differences of the host species, the xenomorphs still preserve core traits found in all variants.

Xenomorphs have a skeletal, biomechanical characteristics, which some theorize means they are an artificially created species, being either black, grey, or bronze-skinned, and no matter the host organism, all variants possess elongated, cylindrical skulls, with no facial features outside of their mouths. Their head possess a distinctive carapace, which can vary, from smooth and translucent to opaque, believed to be part of their maturation process. Because of the DNA Reflex, the flood can appear in both plantigrade and digitigrade forms, according to their host; for example, human, jiralhanae, unggoy and san'shyuum spawned xenomorphs will possess humanoid hind limbs, while flood spawned from sangheili, kig-yar, yanme'e and other animals, will possess hind legs.

The primary weapon of adult xenomorphs is the inner pharyngeal jaw, a piston-like tongue with a secondary set of jaws inside their mouth. It is powerful enough to break through bones and metal, and even energy shields. A common xenomorph killing strategy is restraining its victims with their hands and immobilize them before giving a precise jaw strike directly to the head, piercing the skull and penetrating the brain, known by UNSC as a "headbite". They also have segmented and bladed tails, which varies in size, from small to flat, which make deadly potent weapons capable of impaling and lifting sangheili and jiralhanae, as well as killing a mgalekgolo in two strikes. They possess dorsal tubes on their backs, consisting of two pairs, whose purpose is unknown.

Impregnation
To reproduce and grow their biomass, xenomorphs impregnate, infect, and assimilate other living beings, both sapient and non-sapient, and, despite their capability of using nearly all organisms as hosts, they take preference with the former, as the cerebral tissue of their victims allow them to develop advanced intelligence.

The initiation of a xenomorph outbreak begins with contact with either xenomorph spores or Facehuggers, the latter which are the most complex variant create by the species during their Feral Stage. After seeing a potential host, the Facehugger will jump to its victim's face, rasping clothing and flesh, whereupon it renders the victim unconscious and impregnates them with a Chestbuster. After two minutes, or several hours depending on the stage of the xenomorph, the Chestbuster violently erupts from the host's chest, killing them immediately, and rapidly matures into another variant, shedding its skin and growing, while replacing its cells with polarized silicon. Because of the DNA Reflex, the horizontal gene is transferred from the host to the xenomorph during the gestation period, making the adult xenomorph inherit basic physical traits of the host.

Impregnation is not limited to living beings, as deceased organisms can also become infected, as long as the body has not decomposed to the point of the nervous system deteriorating, Facehuggers can still inject Chestbusters to the body. Using the victim's biological content as a template, the adult xenomorph keeps the memories of its victim intact through isolating the cognitive capability of their brain.

Despite the high efficiency of Faceguggers, xenomorphs can also infect others through the spores, that, if inhaled, transform a host into an Aggressor or other variant, becoming the primary mean of spreading when an infection has reached the Coordinated Stage. They are tiny, and are deployed in a biosphere in large quantities, overwhelming unprotected targets.

Neurological and psychological
Facehuggers selectively attack and impregnate sentient species, being able even to infest intact, wounded, or dead bodies. When Facehuggers attach to the face of their victims, they synchronize with their hosts' nervous system to gain complete control over their motor functions. Any useful information found within the host's memories and brain are then forcibly "taught" by the host to the Chestbuster through the Facehugger, including technical expertise and battle strategies. However, no actual trace of the original mind of the victim remains on the fully grown xenomorph, but merely a very simplistic and intrinsically feral urge to attack and harvest others for infection. These memories and experiences are added to the xenomorphs' whole, being assimilated by the Keyminds and Graveminds.

When xenomorphs are seeking for very specific information in the mind of a host, they use other methods of harvesting, which do not completely subdue the consciousness of their host, even though they are mutated into Proto-Keyminds. The method sees the xenomorphs "burrowing" into the minds of their hosts, which allows them to have an access over all of their victim's memories. This method was utilized against Captain Jacob Keyes, who describe the sensation of the method as being "akin to a rape". Keyes, as a Proto-Keymind, recalled memories from his childhood, his love life, his fatherhood, the location of Earth, and the location of any vessel that could lead them away from Halo to infect other worlds.

After the xenomorphs have acquired enough hosts and biomass to form a Keymind, they reach a coordinated behavior that makes their species much more intelligent. The Keymind is able to telepathically speak to its xenomorph forms, ordering them to fight in unison.

Metaphysical and technological
Although the xenomorphs are biological and biomechanical, as they evolve and reach maturer states, the species transform into a self-replicating swarm able to harness any kind of mechanism to ensure its own survival.

The Logic Plague serve as the biggest meta-biological aspect of the xenomorphs. It is classified as an "abstract harvesting", which is used to subvert non-biological beings, such as the forerunners' Metarchs and Contender, but also potentially synthetics. This "infection" can only occur when a Gravemind is formed, and it has multiple forms, from deep, philosophical persuasion through conversation to aggressive tactics that involve invading the "mind" of the artificial intelligence. This technique was utilized by the xenomorphs during the Forerunner-Xenomorph War to coerce Mendicant Bias into joining them and betraying the forerunners.

The xenomorphs are also capable of using any technology belonging to the hosts it harvested, to a degree even superior to that of the original creators duo to the number of minds they add to their collective, such as with the piloting of the UNSC In Amber Clad.

Sex
The biological sex of the xenomorphs is a heavily debated topic, although it is preeminently proposed they are hermaphroditic, much like the mgalekgolo and the huragok. Some refer to individual xenomorphs by an attributed gender, such as was the case of the Gravemind, referred to as male, and Specimen 6, referred to as female.

Diet
Although little is known about xenomorph diet, the make-up of their blood leads credence to the theory they do not require traditional sustenance like other species. Some engagements between the UNSC, the Covenant and xenomorphs discovered that xenomorphs would sometimes eat potential hosts if the situation was dire. A fact is that the forerunners discovered xenomorph can generate energy high enough so support the species in low or moderate activities, although it did not rule out the possibility of other food sources.

Development stages
Xenomorphs undergo four different stages in their development, which they call 'evolutions':


 * Feral Stage: Consists at the xenomorphs at their most simple, as they are only able to communicate with each other through pheromone secretion. They are sapient, although in a level similar to that of the yanme'e, and are instinctively led to harvest more victims to establish a Keymind.
 * Coordinated Stage: This stage is only possible with the creation of a Keymind, who is able to control all xenomorphs from the first stage alongside new xenomorphs it can form without hosts. This stage marks the Keymind's focus to gather even more biomass and intelligence to become a Gravemind.
 * Interstellar Stage: This stage starts with the formation of the Gravemind and a higher form of sapience. The xenomorphs are able to control space-faring technology, which they use to consume star systems and planets, spreading through the galaxy to harvest more potential hosts.
 * Transgalactic Stage: A final, theoretical stage, where the xenomorphs use all of their captured technology to depart from the galaxy and spread over other galaxies to ensure their continuing existence.

Life cycle
The xenomorphs have a complex life cycle with distinct ages. They start their lives as an Ovomorph, which can be generated either from a Gravemind, Keymind, or a host. These Ovomorphs hatch larval parasitoids known as Facehuggers, which will attack and impregnate any nearby sentient to deposit an embryo known as a Chestbuster. Following the gestation process, which range from hours to minutes to mere seconds, which depends on the xenomorph's stage, the Chestbuster will violently erupt from the chest of its host, leading to the host's immediate death. In another period that can widely range in time depending on stage, the Chestbuster will mature into an adult after replacing its cells with polarized silicon. These adults can fulfill specialized roles, such as Lurkers and Aggressors. As a result of the DNA Reflex, horizontal gene transfer will occur on the adult xenomorph, leading them to possess some of the physical traits of their hosts' species. Once advanced enough, they can evolve to Keyminds and Graveminds, which in turn can freely create new xenomorphs without the need of a host, such as Crushers and Praetorians.

Notable hosts

 * Anne Jorden
 * Bella Clarison
 * Bero 'Kusovai
 * Bok
 * Catherine Foster
 * Ellen Ripley
 * Jacob Keyes
 * Kule Vars
 * Larry Purvis
 * Leslie Orlav
 * Manuel Mendoza
 * Max Gerstein
 * Minister of Etiology
 * Paul Church
 * Prophet of Mercy
 * Prophet of Truth
 * Russ Jorden
 * Spike
 * Thomas Kane
 * Violeta Maldini
 * Wallace Jenkins
 * William Hudson