The Fall of the Eldar occurred well over ten thousand years ago and ended the supremcy of the Eldar race over the galaxy. They 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 destroyed.
Even before this time, it was known that the Eldar would fall. The Old Ones predicted their fall and warned the Eldar of it, but the prediction was forgotten over time.
At the time, the Eldar believed all races to be below them, even so far as calling them barbarians. This proud and arrogant idea was proven by their advanced technology and control of the webways. They created many beautiful things and lived long lives and when they died, their spirits returned to the warp for Slaanesh did not exist then. Slowly, the pride of the Eldar overcame their caution of the galaxy and they became more arrogant. They had long since outgrown the need for physical labour as their society provided all, leaving them with only their own desires to satisfy.
Many gave in to their hedonistic desires and cults sprang up over the Eldar lands dedicated to the different aspects of arcane knowledge and sensual excesses. Gradually, the Eldar society grew more and more divided by the different cults. The people became more and more corrupt, delving into viscious practises, verging on sadism. Sadistic killers prowled the streets in search of victims, attempting to find new ways to satisfy their needs. It became harder and harder to satisfy their desires so the acts became more violent. Soon the streets were running with blood and the beastial roar of the crowds could be heard throughout the empire.
As a result of this foul depravity, a new Chaos god was spawned, Slaanesh, in the image of the Eldar pleasure seeking. No other creature of the warp created had such a violent birth as Slaanesh, or was as powerful or monstrous. For the years before, the Eldar race had been plagued with the images of the sleeping entity. The Eldar slew each other and feasted on the corpses of their fellow kin, while the worlds around them burned. When Slaanesh was born, there was not a single Eldar who didn't feel the pain. With a psionic cry, Slaanesh assaulted the universe and the spirits of the Eldar were drawn from their bodies and sucked into the warp.
The epicentre of the psychic implosion lay within the heart of the Eldar worlds where the Eldar were most concentrated. Most of the Eldar in thousands of lightyears died, their spirits sucked into the warp, and many craftworlds were overtaken. Only those furthest from the centre survived and even most of the Exodite worlds were corrupted by the forces of Chaos and only those furthest away survived. The Eldar were now a doomed race, to be hunted by the Great Enemy for the rest of eternity.
The area that was the centre of the Eldar realm is now the Eye of Terror. It is the largest zone of its type in the galaxy and is continually expanding. The ancient Crone Worlds of the Eldar are now the homes of the Daemon Princes and the Chaos Space Marines turned into hell worlds.