He saw and knew the ancient and evil serpent which swayed there, its wedge-shaped head, huge as that of a horse, as high as a tall man's head, and its palely gleaming barrel rippling out behind it. A forked tongue darted in and out, and the firelight glittered on bared fangs.> - Beyond the Black River
Staring long enough into the eye of the Abysmal Remnant, embedded in this sword, fills the mind with bizarre images.
Once, the Priestking made a pact with a demon from the outer void to inhabit some formed flesh that he was crafting. This was to be a weapon, used in the war against the human city in the north.
The experiment failed, the creature was born limbless and the experiment was abandoned. An acolyte of the Priestking tossed it into the sewer system, to drown in The Dregs.
But the creature did not die, and the demon was not freed. It found a pool where it could excrete the acid that it needed to survive. And then it reached out with its mind, looking for food.
Over time, it grew and the rats and other vermin in the sewers were no long enough to sustain it...