The Underworld is an alternate dimension where all sentient creatures go when they die. It is a dark and twisted place that is made out of islands that are connected by bridges. The rest of the Underworld is just a big, ghostly lake of ectoplasm that the dead call Lake Stykz, "the ocean of the dead". Legend has it that a huge creature named It lives there and drags anyone who should fall into the lake to their doom. Zeath is called "the ruler of the Underworld" and is the only one who can control It. Greffter is the caretaker of the dead, and also the one who keeps the Underworld monsters from escaping to the world of the living, along with Ogenous and Nnoghin. Hades, the leader of the Brotherhood of Darkness, lives here, and has on many occasions tried to become the king of the underworld so that he can release all of the evil Compliens' spirits. There is a secret portal to and from the Underworld and the realm of the living, and one artificial portal, safely in the hands of Greffter, Ogenous, and Nnoghin.
It should be noted that although obtainable, it is not recommended to bring one of these Compliens home if found here. Be warned.
- Castle of the Underworld: Zeath, Rebirtha and Legges's home. L.E.G. Oliver lives hiding in a pocket dimension only accessible from the Castle.
- Thanatostan: The largest country in the Underworld.
- Fango Palace: The Fango family's private property and mansion.
- Mount Tnuom: A gigantic stalactite hanging from the Underworld's ceiling, containing Tnuom Village.
- A mortal spending too long in the Underworld will eventually start losing memories and fade away after a few months. The spirits of the dead fade away too, but take much longer, usually about fifty years.