The “Eye of Terror” is a massive warp-storm caused by the birth of the God / Goddess Slaanesh. This “birth” caused a massive cataclysm which destroyed billions of Eldar and their galactic empire in the process. Slaanesh was in fact formed (or manifested, since it “always” existed) over the course of thousands of years by the negative emotions of the decadent and hateful, but powerfully psychic Eldar.
Morbid Mirrors 