Eight thousand years ago, Lieutenant Commander Raersch undertook a quest on behalf of the Lord Dungard. After slaughtering his way through several opposing armies, he was rewarded with the gift of Daemonhood. After this, several more Marines went on killing sprees, and half of the Chapter descended into anarchy. Dungard put a stop to this by putting a ban on daemonic deification.
As such, Raersch (or Incarna the Hellion as he came to be known) is the only daemon prince who was a battle brother and exalted before his death. Of the whole chapter today, only a handful know of the daemon’s history and true name, and only Takant and Matthawa refer to him as such.
He appears as a typical daemon prince; wings, horns and the rest. His skin shifts constantly between black and red hues, and in battle he manifests himself in a suit of astral power armour. In his right hand he carries the Blade Kirana, a sword of unknown origin which has no trouble lopping off the odd limb or two.
Although Incarna is a daemon, he is able to manifest himself near the Marines of the Legion without trouble. Apparently they give off enough waste energy that he can maintain material form, unless something causes him grief (a good example being a lascannon shot to the gut).