The Cult of Phlegethon
Cutting through the underworld, the river of Phlegathon holds special significance to the Demons who love to bathe in its fiery waters. The cult of Phlegethon believe there is something magical about the waters despite the fact that the explorer and naturist Jamie Baret documented that the waters erupted from a thermal fissure, mixing with natural gas. Jamie even noted that when the waters go out, they are lit by the leader of the cult, a ritual shrouded in prayer and yet more bathing. The cult is not so much secretive as it is in a region where very few adventurers care to tread, so little is known of the cult except that it exists, and its waters are used in The Chop Shop
The cult has an official leader, although in truth the have no power, other than to light the sacred waters , should they ever go out. The true leader is King Hades but he is not the official leader and generally lets the cultists do as they please—unless it happens to clash with his plans.
The cult of Phlegethon is thought to be an important part of demon culture—or it was until recently when demons started to discover other hotpools were more fun to bathe in as adherents to any cult are somewhat creepy, and bossy—which is annoying when all you want when all you want a nice hot soak.
To tend the river Phlegethon and heal the sick. (Most cultists actually hate the sick and are terrified of catching ailments.)