River Styx

This river bubbles with grease, foul flotsam, and the putrid remains of battles along its banks. Any creature other than a fiend that tastes or touches the water is affected by a feeblemind spell. The DC of the Intelligence saving throw to resist the effect is 15.
The Styx churns through the top layers of Acheron, the Nine Hells, Gehenna, Hades, Carceri, the Abyss, and Pandemonium. Tributaries of the Styx snake onto lower layers of these planes. For example, a tendril of the Styx winds through every layer of the Nine Hells, allowing passage from one layer of that plane to the next.
Sinister ferries float on the waters of the Styx, crewed by pilots skilled in negotiating the unpredictable currents and eddies of the river. For a price, these pilots are willing to carry passengers from plane to plane. Some of them are Fiends, while others are the souls of dead creatures from the Material Plane.

Location under

The Outer Planes