Prisoner Zero is one of an inter-dimensional multi-form species, who, having created a psychic link to a living but dormant creature, can then copy the appearance of that creature or of anything they dream about. Coma patients provide the perfect live feed: Prisoner Zero hides in the dreams of the living dead.
The being’s natural form is gelatinous, a snakelike mass of glowing tendrils with a mouth full of sharp teeth. It’s capable of creating a perception filter to stop itself being seen, or to hide anything it wants to hide.
Multi-forms can live for millennia. This one has escaped from being the prisoner of the Atraxi. Prisoner Zero is so dangerous that, to stop it escaping again, they’re willing to destroy the world.