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Acastus (əkăs´təs), in Greek mythology, son of Pelias, cousin of Jason. He accompanied Jason on the Argonaut expedition, but when Jason and Medea murdered Pelias and usurped the throne of Iolcus, Acastus drove them away. Later, his wife fell in love with Peleus, the father of Achilles, who did not return her affection. Enraged, she falsely accused him of raping her. Acastus took revenge by leaving Peleus unprotected on Mt. Pelion. Rescued by the centaur Chiron, Peleus subsequently captured Iolcus and killed Acastus and his wife.