Styx

The river Styx (frequently River Styx) linked the topmost layers of the Lower Planes. The Styx did not extend into the Astral Plane but portals existed along its banks. The seven lower planes, as described by the Great Wheel cosmology model, were Acheron, the Nine Hells, Gehenna, Hades, Tarterus, the Abyss, and Pandemonium. The Styx was also connected to the fifth level of the Nine Hells, Stygia, where it cut a channel through the great frozen sea. Some branches of the Styx flowed down holes in the first layer of the Abyss (which were conduits to the various levels) and some of these holes supplied water to the Styx in reverse waterfalls.

When encountered, the river was a deep, swift, and unfordable torrent. Those who touched or drank from the Styx would forget their past lives completely for a while, sometimes permanently. Travelers were advised to use the boatman Charon or his servants (the charonadaemons) to navigate the river. If nonnatives attempted to circumvent the river by flying or walking over it, there was a good chance of attracting hydrodaemons or styx devils.

The World Tree cosmology model renamed, relocated, and reorganized the lower planes into the Fiendish Planes and the Styx was referred to as the River of Blood.

In the World Axis cosmology model, after the Spellplague, the river Styx became part of the Nine Hells exclusively.