Possessed Marine

Possessed Marines are Chaos Space Marines who decided that just worshiping the Dark Gods was simply not enough, and decided to show their devotion by allowing daemons to enter their bodies. It is worth noting that, unlike in most cases of daemonic possession,the daemon inside a Possessed marine never devours the soul or mind of its host; the two minds instead work in concert to bring as much death and slaughter to their enemies as they can. The bodies of the hosts are wracked with unpleasant looking mutations as the daemons inside gear them towards RIPPING AND TEARING, often growing talons and pincers easily capable of shredding or crushing power armor;yawning maws filled with razor sharp teeth, and copious amounts of tentacles, spikes and horns.

In Black Library books Word Bearer Possessed Marines do not look like a horrible abominations all the time - instead thery transform their body and armor to "battle form" during figt and shift back after. Not much known about is this a unique ability of Word Bearer Possessed, or all Possessed Marines transforms only for battle.