The River Noumenon is an endless river deep beneath RuneScape that separates it from the gates of the 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. He has since been replaced by a different, ghostly ferryman. At the gates of the Underworld, the souls are met by Icthlarin and, previously, Amascut.
The Noumenon is covered by a purple fog and various wandering souls inhabit the area around it. Despite its location, its waters are a clear blue, as with regular rivers on the surface of RuneScape.
River Noumenon is likely a reference to the River Styx in Greek mythology, which serves a similar purpose.
The term noumenon comes from Kantian philosophy, referring to a thing as it is in itself, contrasting with phenomenon, which is a thing as it appears to something else.
^ abThaerisk Cemphier, "Book of the Underworld", RuneScape. "[Icthlarin] then brings the souls he has harvested deep beneath the surface of Gielinor, to the only crossing of an endless river which separates the realms of the living and the dead; the River Noumenon. It is a cold, dark place where a thick veil of fog hangs in the air. This is where Death says he leaves the souls to cross the long, stone bridge over the River Noumenon into the Realm of the Dead."
^Alathazdrar, RuneScape. "I have also worked in the Grim Underworld, escorting the souls of the dead to their final abodes. But the incessant shadows and hellfire weary me, so I have come to serve mortal masters in the realms of light."