Lewis & Short

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,

  1. A. Phlĕgĕthontēus, a, um, adj., Phlegethontian: ripa, Claud. Rapt. Pros. 1, 88.
  2. B. Phlĕgĕthontis, ĭdis, f. adj., Phlegethontian: unda, Ov. M. 15, 532: lympha, id. ib. 5, 544.