The Phlegethon is the River of Fire that flows from Hades' realm down into Tartarus; it keeps the wicked alive so they can endure the torments of the Fields of Punishment.

Interestingly enough though it heals mortals, its fire can kill Cyclopes despite their immunity to fire.

The Heroes of Olympus

The House of Hades

Annabeth Chase tells Percy Jackson that they need to drink from the river because some legends call it the "River of Healing". Percy questions how they are supposed to drink fire. Suddenly, Annabeth thrusts her hand into the river and drinks the liquid. Annabeth starts to shake and gag. After she stopped shaking, her blisters on her arm started to fade. With no warning, Percy passes out and Annabeth desperately gets more fire into her hands and pours it into his mouth. Percy wakes up and makes a comment about how it is "spicy, yet disgusting". He immediately loses his fever. The two use the Phlegethon as a guide to the Doors of Death and continue to drink from the river.

Underworld: DOA Recording Studios | Doors of Death | Door of Orpheus | Elysium | Erebos | Fields of Asphodel | Fields of Punishment | Hades' Palace | Isles of the Blest | Judgment Pavilion | Persephone's Garden
Rivers: Cocytus | Phlegethon | Styx | Lethe | Acheron
Tartarus: The Mansion of Night | Hermes' Shrine | Chaos