Grim Underworld

The Grim Underworld is a world parallel to Gielinor. Apparently, the souls of those who die in Gielinor are ferried to their final resting place, the Afterlife, within the underworld. Each realm has their own underworld, with the Gielinorian one called the "Grim" Underworld. When creatures die, their souls are reaped by Harold Death Esquire, a Guardian of Guthix, hence his nickname "the Grim Reaper", and escorted to the underworld. All underworlds, including the Grim one, have in common that whoever is the first to ever die on that realm becomes the escorter of the dead, as was the case with Harold Death.

The idea of the Grim Underworld contradicts some of those already put forth by other sources, suggesting that only some are taken there, while others appear to travel to another destination.

The Demon butler Alathazdrar claims to have once served as a ferryman on the River Noumenon in the Grim Underworld, but left to pursue other interests in Gielinor and the Infernal Dimensions. This suggests that the Grim Underworld is not necessarily impossible to enter or leave at will, which is further demonstrated with the reincarnation of Robert the Strong into Bob the cat.

Niflheim, only mentioned by name by Drangund, is a place common in Fremennik folklore and religion. According to Fremennik beliefs, those that do not die a heroic death are sent to Niflheim, a world that contrasts heavily with the Fremennik spiritual realm visited during Blood Runs Deep. Due to the Fremennik having strong cultural and religious history and identity, Niflheim is actually just Gielinor's underworld. But they also have their own afterlife, known as the Fremennik spiritual realm..

Trivia

 * The Grim Underworld is similar to, if not the same as, the Hades in Greek Mythology. The reference of Alathazdrar is similar to Charon and the ferry refers to the boat that Charon uses to ship others across the River Styx.
 * Niflheim is the name of a in real-world.