Possessed

The Possessed, known also as the Daemonkin, are Heretic Astartes who have given themselves wholly to the Dark Gods, opening their bodies and souls to be inhabited by daemons of the Warp. This union is no accident but an act of devotion, sought out by those who crave the raw power that only such communion can bestow. In accepting a daemon within them they forsake the last of their humanity, becoming something monstrous and unbound by the limits of mortal flesh.

The daemon's presence twists its host into a nightmare of ever-shifting form. Talons that shear through ceramite, lashing tentacles, membranous wings, and extra eyes and mouths erupt across a body no longer content to keep a single shape. No two of the Possessed appear alike, and each may change from one battle to the next as the entity within reshapes its prison to suit its whims. Warp-fire wreathes their limbs, and their strikes carry a supernatural malice that no mortal warrior could hope to match.

For much of the time the mortal mind endures, sharing the flesh with its daemonic passenger in an uneasy truce. But when the killing begins, the daemon rises to the fore and seizes full command, and the Possessed becomes a whirlwind of savagery beyond all reason or restraint. The first of their kind walked among the Word Bearers during the Horus Heresy, and ever since they have been loosed as living weapons, set upon the foe to butcher without pause until the slaughter is done.