A son of Pelias, king of Iolcus in Thessaly. Strangely he accompanied Jason and the Argonauts, although Jason was an enemy of his father. When the quest was over, Jason had Pelias killed, and then he placed Acastus on the throne.
When Peleus arrived in his kingdom, his wife Astydameia fell in love with him. She propositioned him, and he refused. Astydameia was outraged and sought revenge against Peleus. She went to her husband and told him that Peleus had tried to seduce her. Outraged, Acastus devised a plot to kill the powerful Peleus.
He took Peleus on a trip to Mount Pelion, which was located inside the land of the Centaurs. That night, Acastus and the rest abandoned Peleus in his sleep and hid his sword. Peleus survived, however, thanks the help of Cheiron. Once back in Iolcus , he killed the king Acastus and his queen.
Apollonius Rhodius. Argonautica.