The Underworld is a realm known to be the home of demons and lost souls, souls who once existed as humans but committed unforgivable crimes during their lives. Shinigami send Hollows to hell when they purify them who have committed such crimes during life, as opposed to the ones they send to Soul Society. The Underworld is ruled by the being known as Hades, the only God who doesn't reside inside of Nirvana; in fact, some call the Underworld Hades, naming it after its ruler.