Kher-Ys was an Asuryani craftworld rich in souls that drifted dangerously close to the Eye of Terror. The covetous Dark Prince of Chaos, Slaanesh, and its 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 the daemonic invasion of the craftworld that followed, 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 souls of the Craftworld Aeldari of Kher-Ys for many solar days and nights as thousands fell to their insatiable thirsts. Slaanesh finally claimed its 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 too close to the Eye of Terror, Kher-Ys attracted the attention of the Chaos God Slaanesh, "She Who Thirsts," who coveted it and its rich bounty of Aeldari souls for itself. 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 into the Webway. 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 itself as an Asuryani male and travelled down the Silver River to meet the girl. In her passionate love for him, the Daemon was able to persuade the Autarch's daughter to return with it to her home, where it 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, the 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 it was just starting to wake from the death and destruction being wrought upon Kher-Ys. The Greater Daemon succeeded in taking advatantage of the Avatar's sluggish state and possessed its smoldering body. Led by the newly-possessed Avatar, the Daemons gorged themselves on the souls of the Aeldari of Kher-Ys, and Slaanesh claimed its delectable prize.
Kher-Ys is clearly named after the city of Kêr-Is from Breton myth. The story of the Autarch of Kher-Ys' daughter leading the craftworld to destruction by being tricked by her supposed lover into lowering the craftworld's shields is a retelling of Kêr-Is' myth in which the king of Kêr-Is' daughter steals his keys to open the city's flood gates so that she might meet her lover, dooming the residents of Kêr-Is to drown as the city was consumed by the sea.
- Codex: Chaos Daemons (4th Edition), pg. 26