Demon World

Demon World is a realm where demons are supposedly summoned from in the New World.

Background
A term coined by the inhabitants of the New World to explain the origin of the demonic. According to the Bloody Emperor, the Demon World is supposed to be another plane of existence. The Demon World is alluded many times, though very little knowledge is actually known about the realm.

The Men in the Kingdom Arc
During the Demonic Disturbance, Lakyus Alvein Dale Aindra compared the section of Re-Estize engulfed in flames to a gateway to the Demon World.

The Magic Caster of Destroy Arc
After the Imperial Delegation left the Great Tomb of Nazarick, Jircniv Rune Farlord El Nix believed that his delegation had briefly traveled to the Demon World to the dungeon.

The Paladin of the Holy Kingdom Arc
Neia Baraja was given a towel by CZ2128 Delta to clean herself, wondered about the culture in the Demon World.

Layout
According to Brain Unglaus, when fissures occur, demons from the Demon World can enter into the New World.

Trivia

 * Unlike the other Worlds in Overlord, the Demon World has yet to be confirmed to actually exist.
 * The Demon World is most likely a myth conjured up by New World inhabitants due to their massive lack of knowledge of magical and universal laws (which is typical knowledge possessed by YGGDRASIL players).