Lichfish

Sometimes the will of the fish survives its physical death. Its body rots away, leaving only a sentient, malevolent corpse that swims only through the deepest, darkest waters.

These little horrors are a favourite pet of human (and human-ish) Lichs, as well as other powerful undead such as Mummies, Vampire, Necro Lords and Nomolomancers.

So delvers beware, that innocuous fountain or aquarium in the Dark Lord's lair might hold a terrible surprise.