The only known portal in the Infernal Dimensions
The Infernal Dimensions are a number of worlds and realms parallel to the world of Gielinor. The Infernal Dimensions are said to be the original home of the demon race, which first arrived in Gielinor during the Second Age to serve the god Zaros. According to the Demon Butler Althazdrar, it is the realm where Demon Lords such as Thammaron, Agrith Naar and Delrith came from, and where he served under both before coming to Gielinor to serve mortal masters at the Servant's Guild.
During the quest The Golem, players travel to a small part of the Infernal Dimensions where the Elder-Demon Thammaron once resided, and where he eventually launched a final assault on the city of Uzer in the Kharidian Desert in what would be later referred to as the Battle of Uzer, an important conflict in the God Wars, during the Kharidian Desert Campaign by Zamorak. The small space, which appears to be Thammaron's throne room, may have been part of an abandoned fortress in one of the Infernal Dimensions, although it must be reached via portal and it is impossible to travel any further into the realm from the room.