Backstory: Instead of a normal war between the monsters and the humans, there was a nuclear war. The humans won, and fled underground, leaving the monsters sealed in the overworld, a post-apocalyptic wasteland. The seal used to seal the monsters from the underground was created with flawed magic, as there were very few skilled magicians left. As a result, the barrier is one way, allowing humans to travel to the overworld, but leaving them with no route back into the underground.

Info: The nuclear radiation has caused some monsters to develop physical mutations, while enhancing most of the monsters’ powers by large degrees. Some of these physical mutations include asymmetrical growth, defects in areas such as the pituitary gland,etc. Quite a few monsters died due to radiation, so there are much less encounters in the overworld.  In the underground, humans have developed other soul traits, such as Loyalty, Honesty, and Confidence.  These are the souls of the Nitrotale Universe:

Soul traits other than black are rare, since the hatred towards monsters has been passed down from generation to generation. The hatred poisoned the souls of the humans, and most have either lost their soul traits, are born without one, or have their traits covered by the hate.