Black Oil

A creature composed of prehistoric algae, Black Oil can actually be found in various cases throughout history as the cause for all sorts of disasters, although it was also called under a different name. In the modern area, where drilling for oil has allowed colonies of the symbiotic algae to get to the surface more often then ever, they are now known as Black Oil.
Black Oil attempts to engulf and biologically posses targets, subsuming them on a cellular level. Freeing the captured target before a few days has passed means they can be saved with the proper intervention, but subjects "eaten" weeks ago are long gone.