The Hollow World Connection
by HåvardThunder Rift is also connected to the Hollow World. The main connection is through the dungeons below the Pit, an extremely dangerous place, and it is said that the corridors of that dungeon change like a cluster of serpents (giving it the name the Serpent Caverns), making it even more difficult to navigate. Rumour has it that those dungeons contain a secret path leading to another world.
The rumours are true. The unique nature of the Serpent Caverns allow a quick path to the Hollow World. Unfortunately, the fact that these halls constantly change make it very difficult to use them on a regular basis. Those who make it and survive the horrors within, find their way to the Caves of Ka in the Hollow World, a network of caves leading into the Valley of the Eternal Sun. (See this map from a different thread.
The Valley of the Eternal Sun was recently settled in by time travellers from Thunder Rifts past. The Duke of Barrik (See the Timeline) was kidnapped along with his castle, which used to lay near Melinir when the area was invaded by Goblins. Had not the Immortal Ka intervened, Duke Barrik and his men would have been killed. Instead, they were transported into the Hollow World, where Ka had other use for them. The Valley of the Eternal Sun also serves as a preservation of the time of Thunder Rift's grander past within the Hollow World.
Note: Duke Barrik and the Valley of the Eternal Sun are taken from the Arcade Game: Warriors of the Eternal Sun. The connection with Thunder Rift was hinted at in the module Secret of the Silver Sword in which a Keep named Barrik Keep was featured. I assumed the (now dead) Wizard Barrik who formerly owned the Keep was a descendant of the Duke of Barrik from the Arcade Game. Only ruins must have been left of the Castle, and the Barrik family only rebuilt parts of it forming the new keep, which again eventually was overtaken by monsters (Ratlings).