The Eldar are an ancient humanoid species, biologically and technologically superior to humans. They possess superhuman agility, speed (humans live in a "slow-motion" state to Eldar eyes) and more potent psychic powers. An Eldar army is composed of elite units with specialist roles. They generally favor ranged combat and mobility.
Eldar are relatively benign as Warhammer 40,000 factions come, but usually they don't have much sympathy for mankind and often appear as antagonists. Eldar are featured in all Dawn of War titles from the original Dawn of War to Dawn of War II: Retribution.
It is known that the Eldar were created, or genetically modified from a less advanced species, by an even more ancient race known as the Old Ones, who are also suspected of having created the Orks. Approximately 60 million Earth years previously, the benevolent Old Ones were caught in a war with the Necrons and their C'tan gods, and sought to create several new races to aid themselves. The Eldar were designed to be powerful psykers, for psychic powers were a vulnerability of the C'tan.
The Old Ones were eventually devastated by aftershocks of the psychic powers they had themselves unleashed, but the Eldar remained. They adapted the Webway system created by the Old Ones for their own use, and managed to hold back the Necrons. The numerous unstable Warp connections related to this war ended up causing a wide invasion of Warp creatures known as Enslavers, which feed on the minds of psykers. The sentient populations were decimated, and the C'tan withdrew into "hibernation" in wait of more prey to develop. The Eldar eventually recovered and built the most powerful galactic nation of their time.
All contemporary Eldar are essentially refugees, the scattered remains of a vast interstellar empire. Still, even now they are a force to be reckoned. For millions of years the Eldar were the most powerful race in existence, dominating a significant portion of the galaxy and secure in their prosperity. Even other intelligent races with advanced technology and military power were in no position to seriously threaten the Eldar empire. When it came, their disaster was self-inflicted.
Because an Eldar spirit is reborn into a new body upon death, one individual was able to live countless lives. This, coupled with their already nigh-eternal lifespan, rendered the Eldar nearly immortal, consumed by arrogance and dedicated to the pursuit of their own pleasure. With no need for labor, the Eldar began to pursue their desires with all the dedication that only their species could muster. Cults devoted to exotic knowledge, physical pleasures, and ever-more outrageous forms of entertainment sprang up. It did not take long for many to pursue a darker path to of unbridled hedonism and violence.
Many grew uneasy with these actions, and some of the Seers warned that this path would lead to suffering. Disgusted, some of the Eldar left the central worlds of their empire to settle in the outlying regions of the galaxy, where they built great starships of such size that they could be considered artificial planets; these were the first Craftworlds. Others remained on the homeworlds to try and alter the path their race had taken.
The torture cults eroded the future of the Eldar. While this debauchery would have been destructive within any society, it was even worse for the Eldar because of their powerful psychic abilities and the ability to experience any sensation or emotion more powerfully than any human. Within the Warp, the psychic emanations of these perverse activities began to gather, strengthened by the souls of departed Eldar hedonists and cultists. As the vices grew, this mass of negative psychic energy did as well, producing the terrible Warp storms that defined humanity's Age of Strife and made interstellar travel and communication impossible.
Eventually, in early M31, the psychic energy was born into consciousness as the fourth Chaos God Slaanesh, the Devourer of Souls and the doom of the Eldar. The psychic scream of Slaanesh's birth tore the souls from all the Eldar within a thousand light years of it, sparing only those sheltered within Craftworlds. The cataclysm tore a hole between the physical realm and the Warp, plunging the Eldar homeworlds into a nightmare existence. This region is now known as the Eye of Terror.
Since this time, which is known as "the Fall", the Eldar have been a broken and scattered people, lacking both political cohesion and true purpose. Many of the outlying colonists, known as the Exodites, have slipped to a primitive level similar to feudal human worlds, while others drift through the stars in colossal nomad fleets, each independent of the others.