The Underworld (also referred to as the Netherworld) is the realm in which the dark gods and their minions, the demons and ogres reside. After the first Ogre Battle in which the dark gods were defeated, the armies of the underworld were sealed by the Twelve Disciples and Three Knights. While the ogres are forbidden to travel to the surface world, humanity often contracted with the denizens of the underworld for power and through those contracts the demons are given license to step into the mortal realm.
Eating fruit from the underworld can corrupt beings and eventually turn them into Ogres.