|The following information is from the Adventure Time comic book series.|
It may not be part of the same canon or continuity as the animated series.
The Nightosphere's Nightmareosphere is the dungeon Hunson Abadeer made with the Ice King's help 200 years ago. It's full of demons and monsters and its dungeon boss is Hook Crook. It is so hot that Ice King powers don't work. Somehow, Peppermint Butler has access to the dungeon and writes suspicious and nefarious things on a hollow tree trunk in the entrance.