Underworld

A place deep below the surface of the earth. When the world was created, all creatures that were deemed unfit to populate the surface or the depths of the world were imprisoned in the Underworld by the gods. These creatures are referred to as Demons and they bear a grudge against the gods for the afforementioned reason.

They were put there by throwing them down deep, narrow shafts. Arkath was tasked to seal those shafts with the most durable material in the world: the famed Adamantine