Rictus Claw
Thousands of years ago the VIIIth? Legion, the Night Lords, joined the rebels in their war against the so-called Emperor of Man. The Thramas Crusade saw the Night Lords ravaging Imperial sectors defended by the I Legion, the Dark Angels. Pushed to the brink, the Dark Angels were able to turn the tide on the traitors with the aide of an archaeotech device called the Tuchulcha. The Tuchulcha allowed the Dark Angels to make a sudden and precise Warp jump that left the Night Lords wholly unprepared for ambush. That attack broke the Night Lords as a legion, and the scattered remnants of their forces took to their own paths as renegades, raiders, and interstellar monsters in the aftermath of their Legion's shattering.
But not every Night Lord met that ignominious fate. No, some were more craven still and met an even worse end.
The sorcerer Isshakur Ran was given a vision of the Dark Angels just before their attack. With that vision, he was offered a bargain: he and his ship would be saved but they would have to offer another ship to the Changer of Ways in recompense.
Rictus Claw's ship, the Sword of Absolution, was the ship offered up to Tzeentch. The Sword of Absolution was lost in the Warp, eventually accreting into the Space Hulk that would come to be called the Fulminating Antipathy. For time out of mind, those trapped aboard passed a hellish existence. But now the Fulminating Antipathy enters the Materium again and Rictus Claw is free.
Rictus Claw is free--free except for their one all-consuming need. Each of them is possessed by an undeniable hunger to eat the psykers aboard the Succession--and any others they can get their fangs into.