In the fictional universe of Warhammer 40,000, the Eldar are a race of elf-like humanoids who look into the future via psychic powers. They are one of the most ancient and advanced races in the universe's history, though younger than the Necrons, the C'tan, and the Old Ones. Their armies usually have the advantages of speed and technology, but are often outnumbered and composed of units with little ability to absorb damage. They also have the most Psykers available.
In the sense that Warhammer 40,000 races parallel the earlier (and still extant) Warhammer Fantasy Battle game, the Eldar parallel the Elven race, the "Fantasy Eldar" being the High Elves. In Warhammer 40,000 the Eldar race is divided into three distinct groups, each with its own distinct armies: the Craftworld Eldar (often simply called Eldar), who are similar to the High Elves; the Dark Eldar, who are similar to the Dark Elves; and the Exodites, who are similar to the Wood Elves. (The Exodites exist mainly in Warhammer 40,000 background material and do not have models produced specifically for their use in the tabletop game.) These equivalences are only in very general thematic terms, however, as the major details of the races' backgrounds differ greatly from their Warhammer Fantasy counterparts.
Before The Fall the Eldar were a technologically advanced race, generally considered one of the most powerful races in the entire Galaxy. Their technology had advanced so far that little or no work was required by individual Eldar, and as a result, at some point around the 24th or 25th millennia, groups of Eldar began forming cults dedicated to the pursuit of experiencing everything that life had to offer.
Despite the prediction of the reclusive Eldar Seers that warned of impending doom if the Eldar did not change their ways, government within the Eldar Empire soon collapsed and the moral degeneration of their homeworlds and colonies continued unimpeded. As the pursuit of ever more extreme experiences reached its height, death reigned in the streets of Eldar cities, hunter and hunted each being part of a twisted ritual of destruction which consumed thousands. Some Eldar were able to see that their now-corrupt society was destroying itself, and fled in disgust; these refugees would settle in the distant colonies of their Empire, and would later be known as the Exodites.
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