These creatures are what Dark Vessels transform into, if they successfully summon a demon into their own bodies. Activated Vessels are mutated, half-demonic things who fight furiously and move very quickly. It's a good idea to kill off the Dark Vessels before they can activate, if at all possible.
Activated vessels have impressive death animations, standing still, shaking, and then shooting out embers of red light, like parts of the demon dispelling into the air.
Media
See screenshots and concept art for the Activated Vessel in the Image Gallery.
Activated Vessel concept art.