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.

Cosmology
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.

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 of Blood/Styx flowed through the Abyss and the Nine Hells before dumping its pollution into the Astral Sea.

Geography
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 Styx's headwaters were located in the plane of Pandemonium. There, its deleterious effects were not as powerful as in other layers, so merely touching the water or the spray produced by the plane's intense winds caused no ill effect. While every bit as chaotic and unpredictable as in other planes, the branches of the Styx in Pandemonium were usually shallower.

Rumors & Legends
The silt at the bottom of the Styx was rumored to contain the memories of all the souls who had lost their identities to the river. The shadar-kai were always interested in obtaining the material from adventurers, who sometimes sought to collect vials of the sediment in order to offer it to the Raven Queen in exchange for a boon or divine favor.

Inhabitants
The shallow headwaters of the Styx were inhabited by large versions of freshwater wildlife, such as quippers, eels, lampreys, snails, and crustaceans.