Source: River into Darkness, pg(s). 28-29
Fell flotsam are foul undead oozes that appear as innocuous patches of floating river debris. Created in tropical swamps, they float along jungle-lined rivers preying on any unwary waterway traveler they come across.
Fell flotsam appear like normal patches of flotsam or river debris except they seem to be covered in an oily darkness. They float along on top of the water and are often almost indistinguishable from normal river detritus. One of the things that can give them away is that they can float against the current of the river.
Habitat & Ecology
Fell flotsam are spawned in the same environment they later come to haunt, murky tropical swamps found mainly in the Mwangi Expanse. They spawn when the psyches of many dead, sentient creatures combine in the primordial muck that makes up the swamp. A fell flotsam created by this, while undead, possesses only animal-level intelligence and follows its primary instinct to hunt and kill. A fell flotsam does not actually draw any sustenance from those it kills but they do allow it to slowly grow in size, becoming larger and larger with every kill. Fell flotsam capture their prey by entrancing them with the hypnotic patterns formed on their oily surface.