Netherworld

The Netherworld, or Nether Planes, is a realm mentioned in The Elder Scrolls: Arena.

It is known to be the home of a race of beings known as "Daemons", including Fire Daemons, hell hounds, and homonculi. Powerful wizards may summon these beings into Mundus, often to serve as guards or towards some other service. Some especially power daemons, however, may enter of their own choosing.

Trivia

 * The Netherworld and daemons themselves are mentioned rarely throughout the series, often only as descriptors of other creatures (such as the Nether Lich or Kamal) or items (such as the Demon Tanto).
 * It's likely the concept was a precursor of Oblivion and the Daedra during the early years of the franchise.