Kher-Ys was once an Aeldari Craftworld rich in souls and drifting dangerously close to the Eye of Terror. The covetous Dark Prince of Chaos, Slaanesh, and her minions gathered in a great host to seize their share of the prize.
Leading this daemonic host was the Keeper of Secrets Ail'Slath'Sleresh, known as Heartslayer. During a daemonic invasion of the Craftworld, Heartseeker managed to possess an Avatar of Khaine. The Keeper of Secrets, in the borrowed form of the Avatar, led the Slaaneshi daemon host in an orgy of destruction. They gorged themselves upon the Aeldari of Kher-Ys for many solar days and nights as thousands fell to their insatiable thirsts. Slaanesh finally claimed his prized jewel.
The now empty and desolate Craftworld of Kher-Ys continues to drift along the eddies of the Eye of Terror, its corridors still ringing with the death cries of its people and the laughter of their daemonic slayers.
Travelling so closely to the Eye of Terror, Kher-Ys had attracted the attention of the Chaos God Slaanesh, who coveted it and its rich bounty of Aeldari souls for herself. But the Webway portal of Kher-Ys was protected by powerful runic wards that could be unlocked only by a psychic key held by an Autarch of Kher-Ys, and the Silver River within the Webway that led to the gate into Kher-Ys was regularly patrolled by Asuryani 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 could not resist the lure, and wandered from the sanctuary of their Craftworld. One such Asuryani was the Autarch's maiden daughter, who was bewitched by the Keeper of Secrets named Ail'Slath'Sleresh, better known as "Heartslayer."
The Greater Daemon first disguised himself as an Asuryani 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 into the doomed Craftworld. Daemonettes fell upon the Aspect Shrines, these foul huntresses running 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 of Khaine'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 Aeldari of Kher-Ys, and Slaanesh claimed his delectable prize.
- Codex: Chaos Daemons (4th Edition), pg. 26