Phlegethon is one of the rivers of Hell. Great Marquis of Hell Shax was killed by this river during the revolt of the slave demons.
It was later revealed that the embodiment of the river was with Hellboy when he arrived in Hell. After Edward Grey distracted Eligos who was intent on killing Hellboy, Phlegethon took hellboy to and empty Pandemonium. Phlegethon then gifted the half-demon the dagger which he used to kill a sleeping Satan.
Later after Eligos was killed by Edward Grey who became the embodiment of the river Acheron, Phlegethon, along with Cocytus, Styx, and Lethe welcomed their newly ascended brother inviting him to join them.