The Collaborative International Dictionary
Phlegethon \Phleg"e*thon\, n. [L., fr. Gr. ?, prop. p. pr. of ? to blaze.] (Class Myth.) One of the principal rivers of Hades, in the channel of which fire flowed instead of water.
Fierce Phlegethon,
Whose waves of torrent fire inflame with rage.
--Milton.
Wikipedia
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.
Usage examples of "phlegethon".
It has got me into more hot water than yearly flows through Phlegethon, the famous River of Fire in Pandemonium Park, not three miles from where I sat.
The last time, O Congrio, that I gave a banquet to my friends, when thy vanity did so boldly undertake the becoming appearance of a Melian crane—thou knowest it came up like a stone from AEtna—as if all the fires of Phlegethon had been scorching out its juices.