Ail'Slath'Sleresh HeartSlayer, simply known as the HeartSlayer, is one of Slaanesh's Keepers of Secrets. He took over and controlled the shell of an Eldar Avatar.
When Slaanesh came to desire the CraftWorld Kher-Ys for himself, Heartslayer was one of the daemons who had assembled for the conquest. Because the Craftworld's [WebWay Portal was protected by powerful wards, unlockable only by a psychic key held by one of the Autarchs of Kher-Ys, the daemons instead attempted to seduce those within with their enchanting voices. Many Eldar ignored the siren call, but there were some who fell for it, and wandered from the sanctuary of their Craftworld. One such Eldar was the maiden daughter of the Autarch who held the key. Heartslayer came to her in the guise of an Eldar male, and succeeded in bewitching her. Swathed in her passionate love, Heartslayer was able to pass through the barriers of Kher-Ys. In time, he convinced her to take him to her home, where he stole her father's key whilst she slept, and opened the way for Chaos.
The Daemons of Slaanesh poured through. Daemonettes fell upon the Aspect-Shrines, Huntresses ran rampant through the arcing forest domes, and The Masque of Slaanesh weaved her deadly dance with the Howling-Banshees, ever eluding their blades. In the chaos, Heartslayer was able to gain entrance to the Avatar of Khaine's Chamber, who was just beginning to awaken from the death and destruction being wrought upon Kher-Ys. Blazing into life, the Avatar seized Heartslayer's throat in its unbreakable grip. With a laugh, Heartslayer sheared off the Avatar's hands, and blinded it with swift strikes to its burning eyes. Since the Avatar had only newly awakened, it was soulless; and Heartslayer was able to possess it, and proceeded to lead the Daemons to victory over Kher-Ys in the form of the Eldar's corrupted demigod.