Acephalos

The acephalos is a giant leech that lives on the blood of every sort of living creature. It is not common, but it turns up across much of the known world in swamps, jungles, brackish water, underground pools and even city sewers. Soft-bodied and revolting, it looks like an enormous maggot, its sickly white skin blotched with yellow or brown. Adults can reach six feet, though most are closer to four.

Its jaws are its defining feature. Several rings of sphincter-like muscle drive them, and inside sit two rows of black fangs that curve back toward the gullet, so that when the jaws clamp shut the fangs work like barbs. Though it has no bones or cartilage at all, it moves quickly in water and usually strikes from beneath the surface, often catching its prey unaware; it gains surprise 85% of the time.

Once it bites, nothing breaks its grip. It lets go only when it has drunk its fill, typically after five to ten minutes, and even killing it mid-meal leaves the jaws locked in place. Cutting it free takes a sharp knife and inevitably wounds the victim further, usually for another 1-3 points of damage. The bite leaves an ugly round scar and has a 65% chance of passing on a deadly blood disease.