In the 41st Millenium, there is a race on the verge of extinction. The birth of the fourth major god of Chaos, Slaanesh, destroyed the race that had in its folly created it, tearing a hole thousands of light years wide between normal space and the Warp, now known as the Eye of Terror.
Trillions of lives and souls from all the core worlds of this race were siphoned into Slaanesh's maw, as the Chaos god of pleasure, pain and excess laid claim to all the souls of his creators, slaying all their own gods save two. This was the Fall.
The Eldar Fall brought about the ascention of Mankind, a disgusting race that overpopulated thousands of worlds, then millions as their Imperium took shape. The surviving Eldar were refugees, former traders from their now-broken and destroyed great empire. A fragmented people who struggled on onboard planet-sized ships called Craftworlds.
They watched this savage, barbaric race of men conquer world after world. Worlds that were once theirs. Tirelessly, they now fight against the tide of inevitability, surviving as best they can beset by endless enemies, from mankind's Imperium, to the forces of Chaos, to the hordes of other alien races that had fought the Eldar since time immemorial, now hungry for the blood as they sensed the wounded people.
Ever vigilant against Slaanesh, their Great Enemy, the Eldar built Spirit Stones to trap their souls after death, denying Slaanesh the power it would gain by devouring them. They began the multi-tiered lifestyle called the Eldar Path, mastering one facet of life at a time, one profession, one art. Never again would they fall to the excesses that caused their great failure.
Master psykers, the Farseers, oversaw their survival, watching the skeins of probability and fate to choose the best course of action. Mighty aspect warriors studied the many paths and aspects of their bloody-handed war god, Kaela Mensha Khaine, protecting their people and fighting for the betterment of all Eldar.