The Underworld is one of the Realms of the Gods, the realm of Grenth, god of death. It is a very advanced area and mobs are very strong there.

After being granted access to the Underworld by the Voice of Grenth, you will find yourself next to several ghosts (Gwen's mother Sarah, among others), one of whom will offer you the quest Clear the Chamber. It is recommended to first kill all monsters in the surrounding area before accepting this quest. As soon as you accept the quest several doors will open, allowing you to further explore the Underworld (after you have defeated the hordes of Grasping Darknesses that will swarm you as soon as the doors are opened).

The Underworld is one of the few places where you can find Storm Bows, Ghostly Staves, Eternal Bows, Eternal Shields, Eternal Blades and the highly coveted Globs of Ectoplasm.