River Noumenon

The River Noumenon is an endless river that separates Gielinor from the gates of Grim Underworld. When people die, their souls are reaped and brought to the river by Harold Death. From there, they used to be ferried over the river by Alathazdrar, the demon butler, but this is no longer the case. At the gates of the Underworld, the souls are met by Icthlarin and, previously, Amascut.

Trivia

 * River Noumenon is likely a reference to, which serves a similar purpose.