The Netherworld (also known as Nether-Region, Underworld, Ghostworld, Spirit World, Netherdimension, Nether Real, and the Other Side) is a dimension inhabited by ghosts, demons, and miscellaneous paranormal entities. The Netherworld also is not a single fixed world either being theoretically capable of infinite configurations. It's form is apparently influenced by the mortal world where the two intersect with the Netherworld segment attached to the Public Library having similar contents.
Ghostbusters: The Video Game
After Egon Spengler, Ray Stantz, and the Rookie trapped Eleanor Twitty, a cross-dimensional portal appeared in front of them Ray admitted it was something he's dreamed about since college and mentioned it is all described in the "Monarchia Daemonum."  Upon using the portal, the trio was then taken to the Ghostworld, a Gozerian dimension inhabited entirely by ghosts. While attempting to find their way out of the Ghostworld, the trio traveled through a strange alternate version of the New York Public Library that included a number of various portals through which they ended up getting scattered. After traveling through a series of portals, they found themselves in the Ghostworld's version of the Public Library's Reading Room. They are unable to leave because of Azetlor's hold on them. However, after they defeated him, they were able to leave the Ghostworld and returned to the real world, arriving back in the Reading Room of the New York City Public Library.
Secondary Canon History
Ghostbusters: Sanctum of Slime
- On page 14 of Ghostbusters Volume 2 Issue #4, the portrait of Dumazu's dimension is between Winston Zeddemore and Special Agent Melanie Ortiz.
- On page 4 of Ghostbusters: Funko Universe, Master Takeda mentions the Netherworld.
- On page 14 of Funko Universe, Master Tomo mentions the Netherworld.
Secondary Canon Appearances
- IDW Comics