The Dark Claw was the last remnant of Be'lakor's physical form, severed when Tzeentch struck down the Daemon Prince, stripping him of his power, flaying his mind with such torment as to drive him mad. It was through the Dark Claw that Tzeentch anchored Be'lakor, whom had become a shape without form, a wraith without substance, forced to wander the world forever cursed and reviled. Through the Dark Claw, Be'lakor was chained and bound to serve whosoever held it, the power to command a being that even daemons fear and dread.[1a]
Warhammer Wiki