Ail’Sleresh, The Heartslayer
This is based upon a great story from the Chaos Daemons codex. Essentially (for those who haven’t read it), the story goes that a keeper of secrets tricked an eldar maiden into letting it enter past the protections of an eldar craft world. Stealing the key to the defences, the keeper of secrets deactivated them and let thousands of slaanesh daemons attack the craftworld.
The Heartslayer entered the chamber of the young king within the craftworld before the avatar could be awakened, and woke it as a soul-less monster. Gouging out its eyes and cutting its bloody hand off, the daemon took over the avatar’s form, and so the craftworld was left as a dead world, a testament to slaanesh.

Heartslayer: 270 pts

WS BS S T W I A Ld sv
8 4 7 6 4 10 6 10 4+ invun

Unit type: Monstrous creature
Daemonic Gifts: Aura of Acquiescence, Daemonic gaze, Tranfixing gaze
Special rules: Daemon, Avatar form

Avatar Form: Ail’sleresh has stolen the body of an eldar avatar. The power of this body invigorates the greater daemon. The daemon has the gift Iron hide. Heavy flamers, flamers, melta weapons and other heat based weapons count as being at half strength rounded down and count as ap-.

I was tempted to do an extra rule or something about it being demoralising to see a possessed avatar but i decided not to in the end.