A hydra is a fearsome amphibious monster with five heads.
Paraphrased from “A Recorde of Magickal Beastes of the Mortall Plane”:
The hydra has remarkable regenerative capabilities, second to no other creature of the mortal plane. The secret to its regeneration is believed to lie in its blood, a frightening green liquid that bubbles and seethes with vigorous chemistry. When the hydra is wounded, its blood seeps forth and spurs exponential tissue growth, potent enough to grow two heads where there once were none.
For centuries, scholars have attempted to put the hydra’s blood to use in medicinal practices. Unfortunately, while its blood is of great use to the hydra, it is toxic to humans and the similar races, and these experiments have largely been failures.
The origins of the hydra are unknown.
The hydra hunts similarly to crocodiles, Water Wolves, and other semi-aquatic predators; that is to say, it hunts by ambush, lurking underwater with one head watching in each direction for prey. When a head spies a prey animal, it snaps up the offending creature, and the hydra’s remaining heads lash out to cut off the beast’s escape. The hydra then drags its prey underwater to drown and devour it.
The Darkwater Witches captured and charmed a hydra, which they used as a guard monster outside the Hanging Tree. Davda was responsible for training and guiding the hydra to attack intruders. The party broke the witches’ control over the hydra, and it retreated into the wilderness.