Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Phlĕgĕthon, ontis, m., = Φλεγέθων (burning, blazing), a river in the Lower World, which ran with fire instead of water: Chaos et Phlegethon loca nocte silentia late, Verg. A. 6, 265: Tartareus, id. ib. 6, 551; Stat. Th. 4, 522: ardenti freto Phlegethon harenas igneus tostas agens, Sen. Thyest. 1018; id. Oedip. 162; id. Agam. 752.
Hence,