Styx

The River of Blood was a cross-planar feature in the World Tree cosmology of the planes.

It was a huge and stinking river that flowed through many of the fiendish planes, connecting them similarly to how the World Tree connected the celestial planes.

The source of the River of Blood was thought to lie somewhere in the Abyss. The river ran strongest through the Abyss, Blood Rift, and the Nine Hells. It also wound its way through the Barrens of Doom and Despair, Clangor, Fury's Heart, Hammergrim, and Nishrek, and ultimately poured out into the Fated Depths. The only fiendish planes not connected by the River of Blood were the Supreme Throne and the Demonweb Pits.