Phlegyas is a fiend in Final Fantasy X encountered in the sewer of the Via Purifico. Being a Water-based monster, it is vulnerable to Lightning attacks. After being physically attacked, it will begin to charge, and on its next turn will unleash Sonic Wave on the party. If Phlegyas is put to Sleep while glowing, it will have to charge again.
Phlegyas will drop armor with MP +10% and Auto-Med abilities, and weapons with Magic +5% or Strength +5%.
In Greek mythology, Phlegyas was the king of the Lapiths, as well as the father of Ixion. He was killed by Apollo after burning the temple at Delphi and was condemned to act as the ferryman for souls to cross the river Styx in the underworld.