Definition of "Underworld"

In Scandinavian mythology the dragon is the guardian of the Underworld on the Norse sacred tree, which is the realm of departed spirits, elementals, elves and faery folk ruled by the goddesses Hel and Freya.

1985, Michael Howard, The Wisdom of the Runes, Rider & Company, page 129

At Imbolc, the goddess is transformed from her aged, winter aspect as the Hag, veiled in black, carrying her black rod of barreness as Hela, guardian of the mysteries of death and the Underworld.

2002, Nigel Pennick, Practical Magic in the Northern Tradition, Thoth Publications, page 36

I know that the centre of this labyrinth is keyed to the Underworld, the Land of the Dead, but that's not my goal.

2006, Raven Kaldera, The Pathworker's Guide to the Nine Worlds, Asphodel Press, page 1