Gehenna

Gehenna (ゲヘナ, Gehena; Hebrew for "Hell") is a large, underground kingdom at an unknown location within Earth Land. It is a large kingdom that consists solely of Demons of various kinds, living away from humanity after retreating at the aftermath of the Dragon Civil War. It does not house all of Demon kind, as there still exists some who have left, or even do not know of Gehenna's existence.

Gehenna's existence is considered an equivlant of a myth among humanity, since Demons aren't believed to possess a functioning society like humans do, while others believe in its existence.

Beasts of Gehenna
Beasts of Gehenna (ゲヘナの獣, Gehena no Kemono lit. Beasts of Hell):

Trivia

 * n Jewish Rabbinic literature, and Christian and Islamic scripture, Gehenna is a destination of the wicked.