The Underworld (
The Underworld appears to be a modern town with a large population, similar to the mundane world.
- Big Boss Scree
- Fuzzy's Doppelganger
- Capybara Yōkai
Not anyone can come in and out of the Underworld. Various rules are to be followed by those who want to go in or out.
- Yōkai, and humans (and presumably other things) cannot go to the Underworld straight from the Mononokean if they came into the Mononokean from the mundane world. Likewise, one cannot go to the mundane world straight from the Mononokean if they came into the Mononokean from the Underworld.
- At first, only the master of the Mononokean can travel between the mundane world and the Underworld through the Mononokean. This was changed by Rippō to include the employees of the Mononokean.
- One may only pass between the Underworld and mundane world using Underworld portals.
- Yōkai traveling from the Underworld straight to the mundane world through a portal must secure travel permits, which is costly and involve lots of work.