Styx

The River of Blood is a cross-planar feature in the Forgotten Realms' Multiverse. It's a vast fetid river that flows among most of the fiendish planes connecting them similar to how the World Tree connects the celestial planes.

The source of the River of Blood is said to lie somewhere in the Abyss. It flows most strongly through the Abyss, Blood Rift, and the Nine Hells. The River of Blood also winds through the Barrens of Doom and Despair, Clangor, Fury's Heart, Hammergrim, and Nishrek before finally spilling into the Fated Depths. The only fiendish planes untouched by the River of Blood are the Supreme Throne and the Demonweb Pits.