The river Phlegethon is one of the five rivers in the infernal regions of the underworld, along with the rivers Styx, Lethe, Cocytus, and Acheron. It is described as "a stream of fire, which coils round the earth and flows into the depths of Tartarus". It is parallel to the river Styx. It is said that the goddess Styx was in love with Phlegethon, but she was consumed by his flames and sent to Hades. Eventually when Hades allowed her river to flow through, they reunited.
The Phlegethon is located within the Seventh Circle of Hell, which punishes those who committed crimes of violence against their fellow men; murderers, tyrants, and the like. The portion of the Phlegethon within the Seventh Circle is turned into boiling hot blood instead of fire. By causing hot blood to flow through their violent deeds in life, they are now sunk in the flowing, boiling blood of the Phlegethon.
The depth at which each sinner must stand in the river is determined by the level of violence they caused in life. Examples such as Attila the Hun and Alexander the Great up to their eyebrows. Centaurs patrol the circle, firing arrows at those who try to rise above their allotted level in the river.