This guy is a Herald of Khorne (because they're such cheap HQ choices!), who has been formed from the violent thoughts of Eldar. The perversions of the Eldar gave birth to Slaanesh, and now the Eldar have an entire way of life that attempts to deny Slaanesh by entirely focussing on other pursuits. But surely an entire slice of their population being that dedicated to the Path of the Warrior must have some sort of echo in the Warp? I've always liked the idea that there might be links between Khorne and Khaine, as well.
Obviously the body of the model is the new Exarch. He was tilted forwards slightly, given arms from Dark Elves and Dark Eldar, a head from the Chaos Chariot, and wings from the Vampire Counts Morghast. It's an odd concept, but was quite fun to make and I was worried that I'd never find anyone suitable for the Chaos Chariot head! It's too elegant to be put on a beastman/cultist/marine body.