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. The river is so hot that it's actually cold to the touch.
Interestingly enough though it heals mortals, its fire can kill Cyclopes despite their immunity to fire. That being said; it is believed it can burn or heal whoever the fire god decides to.
Nico leaves Camp Jupiter before Hazel to see if he can locate the Doors of Death from inside the Underworld. During his search, Nico (believing he could go anywhere in the Underworld) comes too close to Tartarus and gets pulled in by the dark gravity the pit generates (similar to a black hole). He was presumably forced to drink liquid fire from the Phlegethon in order to survive. At some point during his horrifying journey through Tartatus, Nico encounters the goddess of misery Akhlys, who was impressed at how much sorrow and pain the son of Hades carried, claiming that there was little more that she could do to him. Though he fought back, Nico was eventually overwhelmed and defeated by the sheer number of Gaea's forces down in Tartarus.
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.
- Despite being a river of fire, not water, Percy was able to control it. Additionally, it's flames affected Cyclopes, who should be immune to fire.
|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:||Mansion of Night | Hermes' Shrine | Chaos|