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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.
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 of Hell, which punishes those who committed crimes of violence against their fellow men (see Canto XII, 46–48); murderers, tyrants, and the like. 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; Dante sees 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. Dante and Vergil cross Phlegethon with help from Nessus.
In Paradise Lost (II, 580) John Milton names the Phlegeton (sic) as one of the rivers of Hell, which bold adventuring demons explore while Satan's flight to Earth begins. Milton also mentions the Rivers Styx, Acheron, and Cocytus. The river Lethe is also counted among the rivers of the underworld.
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.
Track 5 of The Residents' 2011 CD The Rivers of Hades is called "Phlegethon River".