The Fall of Kher-Ys
Kher-Ys attracted the attention of the Chaos God Slaanesh, who coveted it for himself. But the Webway portal of Kher-Ys was protected by powerful runic wards, unlockable only by a psychic key held by an Autarch of Kher-Ys, and the Silver River that led to the Gate was regularly patrolled by Eldar warriors. Unable to enter the Craftworld, the Daemons of Slaanesh instead gathered around it, and attempted to seduce those within with their enchanting voices. Most ignored the siren call, but some fell for it, and wandered from the sanctuary of their Craftworld. One such Eldar was the Autarch's maiden daughter, who was bewitched by the Keeper of Secrets known as Heartslayer. The Greater Daemon then disguised himself as an Eldar male and traveled down the Silver River to meet the girl. In her passionate love, he was able to persuade her to return with him to her home, where he stole her father's key and opened the wards upon the Webway.
The Daemons of Slaanesh poured through. Daemonettes fell upon the Aspect Shrines, Huntresses ran rampant through the arcing forest domes, and The Masque weaved her deadly dance with the Howling Banshees, ever eluding their blades. In the chaos, Heartslayer was able to enter the Avatar's chamber, where he was just starting to wake from the death and destruction being wrought upon Kher-Ys, and possessed him. Led by the newly-possessed Avatar, the Daemons gorged themselves on the Eldar of Kher-Ys, and Slaanesh claimed his prize.