(Greek Φλεγέθων, "flaming," or Pyriphlegethon Πυριφλεγέθων, "fire-flaming") One of the rivers of Hades in Greek Mythology.
"Beach the vessel beside deep-eddying Okeanos and pass on foot to the dank domains of Haides. At the entrance there, the stream of Akheron is joined by the waters of Pyriphlegethon and of a branch of Styx, Kokytos, and there is a rock where the two loud-roaring rivers meet." - Homer, Odyssey 10. 513
"[The] streams are many and great and of all sorts, but among the many are four streams, the greatest and outermost of which is that called Okeanos, which flows round in a circle, and opposite this, flowing in the opposite direction, is Akheron, which flows through various desert places and, passing under the earth, comes to the Akherousian lake... The third river [the Pyriphlegethon] flows out between these two, and near the place whence it issues it falls into a vast region burning with a great fire and makes a lake larger than our Mediterranean sea, boiling with water and mud. Thence it flows in a circle, turbid and muddy, and comes in its winding course, among other places, to the edge of the Akherousian lake, but does not mingle with its water. Then, after winding about many times underground, it flows into Tartaros at a lower level. This is the river which is called Pyriphlegethon, and the streams of lava which spout up at various places on earth are offshoots from it. Opposite this the fourth river issues [the Styx] . . . it passes under the earth and, circling round in the direction opposed to that of Pyriphlegethon, it meets it coming from the other way in the Akherousian lake. And the water of this river also mingles with no other water, but this also passes round in a circle and falls into Tartaros opposite Pyriphlegethon. And the name of this river, as the Poets say, is Kokytos." -Plato, Phaedo 112
“The fire of Phlegethon and the water of Acheron cross in the Ninth Sphere and form the sign of the infinite.” - Hilarion IX