Oni no Chouten

Him is seemingly the God of all Demons and the Demon World. However, his real name can’t be translated. He lives in a special dimension where the laws of time and physics do not apply; said to be located in a mysterious space with a dragon. Anyone who tries to contact Him must need to go through a special ritual and trial that involves strange metaphysical puzzles.

Spoiler

He is the one capable of making promises in this world.