Chasm Spawn

Chasm Spawn are amphibious creatures that crawl out of the Black Chasm in Bretonnia and then wander under the city of Bastonne, attacking and trying to eat anything in their path, including Humans, horses, rocks, and trees. They seem to be almost mindless, but they are dangerous all the same.

They look rather like enormous frogs and can leap long distances, though they prefer not to. Each is about ten feet tall at the shoulder, larger than many peasant houses. Their skin is black and tough rather than slimy.

Young Chasm Spawn have never been observed, but the stability of their shape means that most scholars believe they have nothing to do with Chaos. A few scholars would love to sponsor an expedition to the depths of the Chasm to find out more, as long as they don't have to go along.

Fan Theory

As Black Chasm is a Skaven burrow that has tunnels that lead all the way to Lustria,and whom has Clan Moulder presence, a Theory as to what the Chasm Spawn are is a Skaven experiment on the Lizardmen Spawning Pools. The Chasm Spawn, in this theory, are a type of mutated Slann, as stupid as their normal kin are intelligent, as physically mighty as their normal kin are frail and flabby. Perhaps Clan Moulder poured Mutagens and Warpstone into a Lizardman Spawning Pool, and it started producing these monsters instead, which devour their way through the Skaven tunnels beneath the seabed all the way from Lustria to Bretonnia, devouring Skaven before switching their diet to Bretonnians.