The River of Eternal Darkness (a.k.a the river styx and yellow spring) is a dark, straight and colourless river with some figures around it. It flows silently and never loops.
It is the symbol of death, repose, destination, endpoint, and darkness...etc. It records the death of all beings in the form of death imprints which take the form of the dead. If the original self is ever near their corresponding death imprints, those death imprints will try to drag them into the River of Eternal Darkness.
For the Warrior, Sleepless and Corpse collector pathways, the river water of the River of Eternal Darkness serves diverse purposes determined by its utilization, rituals, and compatibility. It can be used for temporarily clearing memories, healing essential damage to the spirit, improving one's spiritual perception, as an ingredient for important rituals, and for unlocking various abilities, among other uses. It can also be used by Beyonders of any pathway to erase all memories and feelings, becoming an entirely new person.
The River also has an oath/binding-contract property which even entities of the highest level cannot break (without dying or something). Even the god of death would slowly erode if in contact with the river.
The full river is actually sealed behind something.
Everything you don't understand about this is a mystery btw.