Aeacus, in Greek mythology, king of Aegina (now Aíyina). He was the son of the nymph Aegina, for whom his island kingdom was named, and the god Zeus. Hera, queen of the gods, angry with Zeus for his love of Aegina, sent a plague that destroyed most of the Aeginetans. Aeacus prayed to his father to change a group of industrious ants into human beings to people his deserted city. Zeus granted his wish, creating a race called the Myrmidons. Aeacus ruled over his people with such justice that after his death he became one of the three judges of the underworld. He was the father of Peleus and the grandfather of Achilles.