The Circus (Spoilers)

Hell, A.K.A the circus was released into the world in episode 5 of Gomathzon when the dwarves dug to greedily and to deep.

Hell is inhabited by dark and vile creatures like fiends of steam, Ochre monsters and fiends of Cinders.

They quickly swarmed through the lower levels of the fortress before the dwarves made an attempt to seal them back away. It seemingly failed after several Demons entered the Fortress itself and killed a Member of Tomud's Lashers.

A second attempt is being made to seal them away.