The Shustuft Crust (Shukrute in French) is the Krosmoz's equivalent of hell, inhabited only by Demons. Due to the destructive nature of Demons, there is little to say about this dimension: after millenia confined there, they have reduced it to a collection of floating rocks and lava with very few known structures and no ecosystem to speak of.
Rushu's castle
Season 2 of the animated series Wakfu provides one of the most detailed look at the Shustuft Crust, including Rushu's fortress, his "Rushu Rush" arena and vast expanses of floating rock. The Demon King drives the point home in episode 9 with comments such as "Who left something to destroy here?" and "Why would I want to invade your world? Because there's nothing left to destroy here!" (translation ours). One character also mentions the smell of sulfur, commonly associated with lava.