Acastus is a figure in Greek mythology, the son of Pelias, king of Iolcus. He sailed with Jason and the Argonauts and took part in the hunt for the Calydonian Boar. After his father was killed through Medea's deception, he became king of Iolcus and held funeral games in Pelias's honor. He is also remembered for his role in the story of Peleus, whom he first purified of murder and later betrayed.
Adapted from Wikipedia, "Acastus" · CC BY-SA 4.0
I.Family
Acastus was the son of Pelias, then king of Iolcus, and Anaxibia (called Philomache in some traditions). With his wife Astydamia (also called Hippolyte, daughter of Cretheus), Acastus had two daughters, Sterope and Laodamia, and a number of sons.
Another daughter, Sthenele, was given by the Bibliotheca as the wife of Menoetius and the mother of Patroclus.
II.Mythology
After the return of the Argonauts, Acastus's sisters were manipulated by Medea into cutting their father Pelias to pieces and boiling them. When Acastus heard of this, he buried his father, drove Jason and Medea from Iolcus — and, according to Pausanias, his sisters as well — and instituted funeral games in honor of his father. He thereafter became king of Iolcus.
III.Peleus and Astydamia
Acastus purified Peleus of the murder of King Eurytion of Phthia. Acastus's wife — variously named in mythology, often Astydamia but sometimes Hippolyte, daughter of Cretheus — fell in love with Peleus, but he scorned her. Bitter, she sent a messenger to Antigone, Peleus's wife and daughter of Eurytion, to tell her that Peleus was to marry Acastus's daughter Sterope.
Astydamia then told Acastus that Peleus had tried to rape her. Acastus took Peleus on a hunting trip and hid his sword while he slept, then abandoned him on Mt. Pelion to be killed by centaurs. The wise centaur Chiron (or the god Hermes) returned Peleus's sword, and Peleus managed to escape.
With Jason and the Dioscuri, Peleus sacked Iolcus, dismembered Astydamia — and, in some accounts, Acastus himself — and marched his army between the pieces. Their kingdom later fell to Jason's son Thessalus.