From PathfinderWiki

Eridanos is an ocean-sized marsh in Avernus formed by a cascade of countless foul arteries of strange fluids which wash a portion of the Maelstrom's refuse into Hell. The multiverse's refuse is mixed with the souls of the damned and the eroded sediments of Hell; then the noxious mixture forms the headwaters of the infamous River Styx which flows through the planes.[1]


  1. F. Wesley Schneider. (2009). Princes of Darkness, p. 6. Paizo Publishing, LLC. ISBN 978-1-60125-189-3