Aeacus was King of the island Aegina whose father was Zeus and his mother the sea nymph Aegina. Zeus jealous wife Hera was so angered by their relationship that she took revenge by sending a plague to the island, killing almost everyone on it. Aeacus then asked his father to turn ants to people so that his island would be populated again, and so Zeus made the myrmidons - the ant people, who were to be the fierces warriors in Greece.

Aeachus had three sons: Phocos, Telemon and Peleus. Phocos was the most loved son, and in a fit of jealousy his brothers killed him. Telemon fled to Salamis, and Peleus to Thessaly. They each had a hero son: Telemon Ajax and Peleus Achilles. When Aeachus died he became one of the three judges of the underworld, since he had been such a righteous king.