The Eldar are a race of Elf or fey-like humanoids, and they are the most ancient and technologically advanced of all the intelligent races of the Milky Way Galaxy, with the exception of the Old Ones, the Necrons and their C'tan masters. Their armies usually have the advantages of great mobility, potent psychic abilities and technology that is more advanced than that deployed by the human Imperium. In the sense that Warhammer 40,000 races are generally derived from the earlier Warhammer Fantasy universe, the Eldar are derived from the High Elves of the Warhammer World, except for the Eldar faction known as the Exodites who are the equivalent of the Wood Elves. All of the Eldar alive today are essentially a refugee population, the scattered remains of a formerly vast interstellar empire. 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 perhaps the 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. When it came, the disaster for the Eldar people 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 and their ability to experience the highs and lows of emotions more powerful than that of any human, rendered the Eldar nearly immortal, consumed by arrogance and dedicated to the pursuit of their own pleasure. Within the parallel dimensional realm of the Warp, the psychic emanations of these perverse activities began to gather, strengthened by the souls of departed Eldar hedonists and cultists. As the Eldar's vices grew, this dark mass of negative psychic energy did as well, producing the terrible Warp storms that defined humanity's Age of Strife and made all interstellar travel and communication impossible for the human colonies of this period.
Eventually, this growing mass of negative psychic energy came into a life of its own and came to consciousness over ten thousand years ago at the end of the Age of Strife as the newborn 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 in the wraithbone hulls of the Craftworlds. The Prince of Pleasure's awakening was so forceful that it tore a hole between the physical realm and the Immaterium, plunging the Eldar homeworlds into a nightmare existence, trapped within the realm of Chaos. 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 Eldar colony worlds, whose people are known as the Exodites, have slipped to a more primitive technological level similar to that of some feudal human worlds, while the survivors of the Eldar homeworlds drift through the stars in colossal nomad fleets and their Craftworlds, each independent of the others.