In Greek mythology, Phegeus was the king of Psophis in Arcadia who purified Alcmaeon after the murder of Eriphyle and gave him a daughter in marriage. When Alcmaeon later retrieved Harmonia's necklace and robe for Callirrhoe, Phegeus' sons ambushed and killed him; Alcmaeon's sons by Callirrhoe then slew Phegeus and his wife. Distinct from other figures named Phegeus.
Adapted from Wikipedia, "Phegeus of Psophis" · CC BY-SA 4.0
I.Family
Phegeus was the son of the river-god Alpheus. He is said to have been the father of Alphesiboea, Temenus, and Axion, or of Arsinoe, Pronous, and Agenor. The town of Phegeia — formerly Erymanthus, later Psophis — was believed to have taken its name from him.
II.Mythology
When Alcmaeon, pursued by the Erinyes of his mother's murder, came to Psophis, Phegeus offered him succor and gave him his daughter Arsinoe or Alphesiboea. She received from Alcmaeon, as a wedding gift, the robe and necklace of Harmonia. Because of Alcmaeon's crime the ground became barren, and an oracle sent him to Achelous for purification; there he took Callirrhoe to wife.
Callirrhoe demanded the necklace and robe. Alcmaeon returned to Psophis claiming an oracle required him to dedicate them at Delphi; Phegeus believed him, but a servant revealed the truth. Phegeus' sons Pronous and Agenor (or Temenus and Axion) ambushed and killed Alcmaeon. According to Hyginus, Phegeus himself killed his son-in-law.
Callirrhoe prayed to Zeus that her sons by Alcmaeon grow to manhood at once; Amphoterus and Acarnan then slew Alcmaeon's murderers and afterward killed Phegeus and his wife in their palace at Psophis.