Khyber

The common creation myth contends that when the Pro­genitor Dragons fought, Eberron trapped Khyber in her coils and became the world, imprisoning her evil sib­ling in a living prison. Bound within the world, Khyber spawns demons and monsters to plague the children of Eberron. This might be myth and metaphor, but it's also a description of fact: there are worlds within the world, realms inhabited by aberrations, fiends, and all manner of monsters.