Book VI of the Aeneid is over 900 lines of dactylic hexameter describing a catabasis to the Underworld. (Out of those 900-some, about 600 lines actually describe the Underworld; the remaining 300 are spent on preparations and travel to get there.) Many English translations are available, but this one stays relatively close to the Latin.
Starting at line 548 is a description of Tartarus, where the wicked are punished for eternity. Mainly it's described as an enormous chasm, twice as deep as the distance between heaven and earth, and in the depths of that chasm the various punishments take place (ranging from Tityus, chained down while a vulture devours his constantly-regenerating liver, to Theseus, stuck to a bench until Hercules eventually frees him). There is, however, some fire:
Respicit Aeneas subito et sub rupe sinistra
moenia lata videt triplici circumdata muro,
quae rapidus flammis ambit torrentibus amnis,
Tartareus Phlegethon, torquetque sonantia saxa.
(Translation mine:)
Suddenly Aeneas looks back, and beneath the cliff to the left he sees fortifications surrounded by triple walls, around which a deep and swift river of roaring fire flows and flings up echoing boulders: the Tartarean Phlegethon.
This description in general is quite heavily inspired by Book XI of the Odyssey, in which Odysseus describes his own necromantic rituals and experience in the Underworld. But the Odyssey has no mention of any sort of fire: Homer's Underworld is a dark, empty, lifeless place.
"Phlegethōn" is a transcription of the poetic Greek word φλεγέθων, "blazing", which is a pretty good indication that this detail came from a Greek source. However, I don't know what this source might have been: Homer mentions in passing a river named Pyriphlegethōn "blazing with fire", but it's never described and Odysseus never encounters it.
