In Greek mythology, Aeacus was the first king of the island of Aegina. He was the son of Zeus and Aegina, daughter of the river god Asopus and the woman for whom the island was named. Zeus used an eagle to carry Aegina off to the island, originally called Oenone, where she bore him Aeacus. Her father Asopus searched for her, asking King Sisyphus of Corinth for help. Sisyphus had seen the eagle and told the river god about it. Zeus was enraged that Sisyphus had informed on him. In the Underworld, the king was forced to roll a rock uphill that would roll back down over him forever.
When he grew up, Aeacus became the island's king. Because of his parentage, Aeacus was hated by Hera, wife of Zeus. She sent a terrible plague or drought that killed many of the island's people. Aeachus travelled to the temple of Zeus. He prayed that Zeus send rain and that he make Aegina's people as numerous as a nearby troop of ants. Zeus sent rain and miraculously transformed the ants into men. These were the Myrmidons, legendary warriors of Aegina named for the ant (myrmex).
Aeacus' sons were Telamon and Peleus. Peleus was a hero and the father of Achilles. Achilles was sometimes named with the epithet Aeacides, "descendant of Aeacus." In the Attic mythological tradition, Aeacus was honored after his death for his famous wisdom. He was made one of the three judges of the Underworld, alongside Rhadamanthus and King Minos. In Greece's pagan period, Aeacus' legendary prayer to Zeus was honored with a temple on Aegina and an annual festival, the Aiakeia.
References
Aeacus (1974). Encyclopædia Britannica, Vol. I. Chicago: Encyclopædia Britannica, Inc.
Hamilton, Edith (1942). Mythology: Timeless Tales of Gods and Heroes. New York: Warner Books.