Dark Samus is a playable character in the fighting video game, M.U.G.E.N Trilogy. She is a darker version of Samus and was confirmed on December 14, 2018 along with the other characters who appear in Super Smash Bros. Ultimate.
When Samus Aran defeated the titular creature of Metroid Prime, it took one last grasp at Samus and, in its seeming death throes, stole her Phazon Suit before it dissolved into a pool of Phazon. After Samus was forced to leave the collapse of the Impact Crater, Metroid Prime had, unbeknownst to her, survived their battle. Some time afterward, the Phazon Suit's hand emerged from the pool of Phazon, albeit with clawed fingers and an eyeball on the back of its palm.
This effectively signaled the rebirth of Metroid Prime as a Phazon-based cyborg, as the creature's essence and remnants of Samus' DNA within the Phazon Suit constituted its biological components, while the Phazon Suit itself constituted its mechanical component by becoming the creature's new body.
Thanks to these traits, the reborn Metroid Prime managed to acquire all of Samus' abilities, as well as human-level cognition. However, the creature was also obsessed with absorbing Phazon, a trait it retained from its time as Metroid Prime.
How to Unlock
Use Dark Samus' Super Smash Bros. amiibo or scan Dark Samus' AR Icon from the M.U.G.E.N Trilogy website.