Fiction:The Firmament/River Styx

River Styx is an immense interplanar river of cursed and damned souls which traverses the Underworld, Ziana and Irkalla. It is the domain of the great god of death, Dominus Mortis, and the psychopomp gravekeeper Charon.

Description
The River, despite its description, is not made up of water; rather, it is made out of the ectoplasmic essence of millions upon millions of souls, considered damned by death itself. The River is born at the Underworld, traverses the edges of Ziana and finally ends in Irkalla, where its contents are poured into the Abyssal End, burning and poisoning the flesh of the condemned god Varagarash Narahil. Theologists believe the River did not always end in such a manner, and was rather altered by Mortis in order to punish Varagarash for his transgressions.

Inhabitants
River Styx itself is devoid of any true life, as attempting to enter it causes one to be attacked by the desperate souls which make it up, eventually causing the intruder to be absorbed into it. However, certain races of outsiders have adapted to hunting in the river, serving as ambush predators and preying on anyone who may approach its edges, though even such creatures must be cautious to not become part of the flow or be lost forever to it.