Eldar

The Eldar are an ancient alien race whose vast empire, the Aeliern once expanded the width and breadth of the known galaxy. Their empire was only rivaled by the Forerunners and their Ecumene, which also compromised the known galaxy, they counting themselves masters of the stars and children of the Xel'naga. But millennia ago, their overweening pride and their fall into hedonistic practices led to a cataclysm instigated by the Flood where their kind were all but eradicated and led to the birth of the Chaos God Slaanesh. Despite their boundless power, the heart of their civilization was torn out by this catastrophe of their own making, forcing the surviving Eldar to flee upon gigantic starships called Craftworlds. Now they cling to survival by a thread, fighting the horrors of the galaxy with ritualised discipline and consummate skill. Though highly advanced and feared across the galaxy, the Eldar are a dying race -- a shadow of their former glory -- their race teeters on the brink of annihilation.

Children of the Stars
The ancient history of the Eldar stretches back over the millennia to a time when they dominated the stars against the Forerunners, who also believed themselves to be chosen to inherit the Mantle bestowed upon the Xel'naga. Yet, for all their splendour and might, a terrible curse was brought upon them that sundered their empire forever, the arrival of the parasictic Flood, leaving the ravaged fragments of their race teetering on the brink of annihilation. All of the Eldar alive today are essentially a refugee population, the scattered remains of a formerly vast interstellar empire. Those were not destroyed by the Halo Array, as they hide themselves into the Forerunners' shield worlds. Even in such straits, however, they are still a deadly and influential force in the galaxy. Once, over ten thousand years past, the Eldar were the second most powerful race in existence, dominating a significant portion of the galaxy and secure in their prosperity. Although there were other intelligent races who possessed advanced technology and potent military power in the galaxy, none were in a position to seriously threaten the state of the Eldar empire other than the Forerunners.