Eldar are an ancient, elf-like race, who once were the dominant force in the galaxy. However, at some time in the past (most commonly conjectured to be thousands of years before the Humans' Great Crusade) the Eldar race experienced a dramatic catastrophe known to them as The Fall. The Eldar's highly psychic nature as a species had caused significant repercussions in the Immaterium, ultimately resulting in the genesis of a new Chaos entity known as Slaanesh. The Eldar's close link with the new god resulted in a massive intrusion of the Immaterium into the material realm (now known as the Eye of Terror) and the instantaneous destruction of many billions of Eldar; the majority of their species. There are four main factions of Eldar:
- Craftworld Eldar: Those Eldar who fled before the Fall on massive ships known as Craftworlds.
- Dark Eldar: Those Eldar who have embraced the fall, and continue to live a hedonistic life in hiding from the Chaos God Slaanesh.
- Exodites: Eldar who fled to the edge of Eldar space before the Fall and have embraced a more primitive lifestyle.
- Harlequins: A mysterious faction of the Eldar, they are highly in tune with the cycles of past and future - acting out stories of the past from before the Fall, often with galaxy-changing consequences. They live within the Webway, are highly adept warriors, and religiously guard the repository of Chaotic knowledge known as the Black Library. Their actions are unfathomable to all but those of their order - even to other Eldar - making them extremely unreliable allies, even to those of their own kin. For the Harlequins, the galaxy is their theatre; its inhabitants the actors - forcing them to repeat the same act millennium after millennium.
In addition, there are Eldar known as Outcasts. These are Eldar who have left their home-world to wander the galaxy, and do not belong to any of the above-mentioned groups. They usually operate as mercenaries, pirates or adventurers. A final remaining faction are the Crone World, or Chaos, Eldar. Chaos Eldar serve Chaos in exchange for the protection of their souls and are some of the most potent servants of Chaos. First mentioned in earlier Games Workshop publications, their role in modern Warhammer is minimal or nonexistent.
Read more about this topic: Warhammer 40,000 Species