From Lexicanum:
The Fall of the Aeldari occurred over ten thousand years ago and ended the Eldar's supremacy in The Galaxy. Before the Fall, the Eldar were at the height of their empire and held sway over the vast majority of the galaxy; their worlds were paradises and full of peace and cultural achievement, and then it was all but destroyed. It is said that trillions of Eldar died in the cataclysm.