The Underworld is a dark realm where deceased lifeforms can become animate. Certain locations can transport people to the Underworld, the most notable one being the lava in the Fire Temple. The Underworld is home to the Skulkin, who share the realm with the Spykor, which live along the ceiling stalactites. Pieces of land stay afloat in midair, connected by long wooden bridges that the Skulkin regularly traverse over. They keep torches lit by each bridge to provide sufficient lighting.
- Ghosts (Anacondrai Warriors)
- Samukai (formerly)
At an unknown point in time, the Underworld came into existence alongside the rest of the other One Trillion Realms, where it would function as the afterlife for fallen warriors and animals who failed to realize or accept their passing, trapped in a state of limbo.
- The Underworld is the only realm with the most entrances to it, after the Living Side.
- The Wicked Skulls is the only way to open the Underworld.
- In The Curse of Nibiru: Part Eight, Colten Potter tells Scott Dyer that they're kind of in Hell, which most likely means the Underworld is a segment of Hell.