In Greek mythology, the river Phlegethon (Φλεγέθων, English translation: "flaming") or Pyriphlegethon (Πυριφλεγέθων, English translation: "fire-flaming") was one of the five rivers in the infernal regions of the underworld, along with the rivers Styx, Lethe, Cocytus, and Acheron. Plato describes it as "a stream of fire, which coils round the earth and flows into the depths of Tartarus." It was 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.
Literary references 
In Oedipus by Seneca the Younger, the first singing of the chorus, which mainly describes the plague that has settled in Thebes, includes the line, "Phlegethon has changed his course and mingled Styx with Theban streams." While this is not essential to the plot of the play, the line figuratively serves to suggest Death has become physically present in Thebes. The line also reveals the common preoccupation with death and magic found in Roman tragedy.
In Dante's Inferno Phlegethon is described as a river of blood that boils souls. It is in the Seventh Circle and is guarded by centaurs, who force souls to remain at their level. Here are punished the shades who committed crimes of violence against their fellow men (see Canto XII, 46-48). Here are murderers and tyrants: men who through their violent deeds in life caused hot blood to flow and now themselves are sunk in flowing, boiling blood. Dante sees Attila the Hun and Alexander the Great up to their eyebrows in it. He gets across it with help from Nessus.
In Edgar Allan Poe's short story "Descent into the maelstrom," the narrator, looking down on the whirlpool from a mountain, refers to the water as "the howling Phlegethon below," signifying its danger and coiling effect.
In the novel Inferno by Larry Niven and Jerry Pournelle, Phlegethon is guarded not by centaurs, but by military officers taken from all eras of history (with instructions to shoot anyone who tries to escape). There is also a wooden sailing ship sunk on the other side, which is inhabited by the souls of slave traders.
In the first arc of the Curse of the Spawn series, the Spawn's creator Plegethonyarre was named after the mythological river.
See also