Demon World (魔界, Makai) is the world where demons live. It is a planet that King Enma sealed off with a powerful barrier which forbade any demon ranked A Class or higher to cross. For many years it was ruled by 3 kings: Toshin Raizen, Mukuro and Yomi, though not in the literal sense.
Each territory had its own individual laws, as established by its rulers, until the establishment of the Demon World Tournament, whose victor would unify Demon World under him/her as the supreme ruler. Raizen, Mukuro, and Yomi are acknowledged as the three kings because out of all the kingdoms in the land, they have the greatest influence and strength to negotiate a deal with King Enma to allow Yusuke, Kurama, and Hiei to return to Demon World.
According to Koenma, Demon World is rather like an endless basement, with its strongest demons living deeper and deeper within its depths, while its surface levels are inhabited by demons of fair power and notoriety. Demons within its ultimate depths are said to be god-like and mere embodiment of destruction to the point where even a single one of these demons could completely devastate the Earth with no one to stop them.
The sky in Demon World seems to be crimson, with dark clouds during daytime. Much of this world seems to be terrains, including untouched wastelands, forests, mountainous regions, deserts etc. However, despite this natural and primitive environment, Demon World's technology surpasses that of Human World, as healing chambers and artificial birth machines have been employed by the three kings on different occasions. Also, Demon World seems to have cities with technologies such as television as seen during the Demon World Tournament.
- Demon City, which is occupied by the Four Saint Beasts, and Hyouga the Island of the Koorime are distinct territories that did not appear to be under direct control of any of the 3 demon kings.
- The Forest of Fools, an area mentioned by Hiei, is a territory where civilian Youkai live peacefully.
- The Plateau of the Beheaded
- In episode 98 of the anime, as Yusuke and Raizen's servants run through the Demon World, a sequence of images show some of its landscapes. The images, depicting unusual lands, are actually illustrations done by Roger Dean, a British painter mostly known for the covers of the albums by the progressive rock band Yes. The illustrations include covers from: Yessongs, Relayer and Keys to Ascension.