The Underworld is ruled with a stern and terrifying manner by the most recent leader Kibosh. The leader's task besides ruling the Underworld is to preserve The Balance between the Human World and the Underworld and they are not above banishing friendly creatures to accomplish this goal.
The Underworld is divided in boroughs for each creature species as revealed in "Master Blister". The ground is a very light shade of pink while the sky and clouds are pinkish purple during the day and at night the sky is dark blue but not as dark as the Human World's. The sea is light green.
Not much of the Underworld was seen besides Scare School and the area surrounding it.
Creatures are forced to master the art of scaring humans as soon as they begin walking. Non-scary creatures are usually forced to attend the Underworld's version of reform school known as Scare School.
The balance is important to the creatures. If they scare humans too little or too much they will rise up against the creatures. It is the task of every creature, especially their leader, to do what it takes to preserve the balance and the punishment for not doing their part on preserving it is banishment to a place that could be considered hell for the creatures: The Valley of the Shadows.
The Underworld is the polar opposite of the Human World. Friendly and kind behavior is considered inappropiate as it leads to creatures becoming soft and non-scary. Human toys are forbidden. Humans are referred to as "Fleshies" and are looked down by most creatures. Tough and scary creatures are held in a much higher standard than friendly ones.
The creatures' diet consists of food that could be considered gross and/or unedible for humans such as Slimonade, eyeballs or rotten apples, although creatures also eat human foods.
The creatures are known to celebrate human holidays such as Christmas and possibly Mother's Day too. Despite this, Dash claimed that creatures don't celebrate holidays that involve being nice. Oddly enough, creatures have no aknowledge of what Halloween is except for Casper.
- Chamber of Unbearable Odors (mentioned)
- Chamber of Unspeakable Horrors (mentioned)
- Creepy College (mentioned)
- Flyville (mentioned)
- Ogre Village (mentioned)
- Penalty Box of Doom (mentioned)
- Petrified Prep Elementary (comics)
- Scare School