River Pyriphlegethon
Flowing from Mount Erebus in Phlegethos, Baator, the River Pyriphlegethon changes in nature along its length. At its source it is pure molten rock, much like everything else in the layer. From there it loops around to Phlegethon in Pandemonium where it is a congealed, nearly solid flow of lava and hot mud. Mind you, it’s still much hotter than the rest of the layer, so a body who needs heat badly could do worse than to go looking for the flaming river.
Sinking next into the uncharted depths of the Abyss, the river’s path is lost. It emerges again in the Hinterlands. Passing by the Dark Forest encircling the Slavic Other World as a wide chasm of flames, it flows along the Long Valley as far as the eye can scan.
Somehow it next flows into Avernus, where the Pyriphlegethon is only distinguishable from the River of Blood by temperature: Pyriphlegethon boils. Having thus gone full circle around the Lower Planes it flows into the Styx at Stenching Evil, the City of Revulsion.
Source: Alex Roberts
As a Slavic person and a Slavic mythology nerd I appreciate having the river of fire in the Slavic Otherworld. It’s a lesser-known detail
(Yeah, I know it’s from Tiefling’s Exultation)