[3] 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.
In Dante's Inferno, which is the first part of Divine Comedy, 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.
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.