Phayllus was the third leader of the Phocians during the Third Sacred War, succeeding his brother Onomarchus. After a fairly unsuccessful period in command he died of natural causes, and was succeeded by his nephew Phalacus.
Phayllus served as a general before coming to power. His brother gave him command of a 7,000 strong army that was sent to Thessaly to support Lycophron, tyrant of Pherae against Philip II of Macedon. Phayllus was defeated by Philip, and Onomarchus was forced to intervene in person. He was more successful, and inflicted two very badly documented defeats on Philip. This was only a temporary setback for Philip. In 352 BC he led his army back into Thessaly and defeated the Phocians and their allies at the battle of the Crocus Field. Onomarchus was killed after the battle.
Phayllus succeeded his brother as leader of the Phocians. In the aftermath of the destruction of their army at the Crocus Field he had to spend a large proportion of the treasures of Delphi. According to Diodorus he had turned 120 gold bricks donated by Croesus, 360 golden goblets and golden statues of a lion and of a women into coins worth 4,000 silver talents, and had also spent part of 6,000 talents worth of silver. This allowed him to double the pay he offered to mercenaries. He was also sent reinforcements by the Achaeans (2,000 men), Athenians (5,000 infantry and 400 cavalry) and Spartans (1,000 men). Finally the defeated tyrants of Pherae, Lycophron and Peitholaus went into exile in Phocis with their remaining 2,000 troops.
Phayllus was less successful as a general. His first campaign took him into Boeotia, where he was defeated in three battles. At Orchomenus he lost 'a great number of men', at the Cephisus River he lost 500 dead and 400 prisoners and near Coronea he lost 50 dead and 130 prisoners.
His next campaign was an invasion of Epicnemidian Locris, where he was able to take most of the city, but failed at Naryx. He captured the place by treachery one night, but was then expelled having lost 200 men. He was then attacked by the Boeotians at Abae, and lost yet again. The Boeotians then carried out a raid into Phocis, before moving to the 'city of the Narycaeans', which was then being besieged by the Phocians. Phayllus's military career ended with a rare success. He appeared unexpectedly outside the city, drove off the Boeotians, captured the city and razed it to the ground.
Phayllus died in 351 after a long illness, which his enemies saw as a divine punishment for the Phocian seizure of Delphi. He was also accused of giving some of the treasures to his wife and mistresses,
Phayllus was succeeded by his nephew Phalacus (the son of Onomarchus), although at first the young Phalacus ruled alongside one of Phayllus's friends, Mnaseas. Phalacus wasn't much more successful in battle, but he did manage to survive the war, eventually dying in exile, probably in 338 BC.