Yomi (YI Arc)

Yomi is a spirit world adjacent to Hell and resides as the home of the Yōkai as well as the world containing the location of Maou's former prison, to those viewing it from the outside it appears to be a black hole-like sun that is absorbing all the light and colour in the Universe, Yomi is in a constant state of self destruction - destroying and repairing itself - but only stops when its opposite (the Human world and Spirit world) are in conflict and chaos.

Yomi has its own Imperial Government that is ruled by the Eight Yōkai Emperors who go about their day-to-day business of striking out against the living world and Soul Society, they have begun to gather the makings of an an army - the Army of the Destroyer - and intend to take their war directly to the living world and other spiritual worlds, to conquer and to cause as much chaos as possible.

Trivia

 * Yomi is the name of the Japanese underworld.