Acastus is the son of Pelias and Phylomache and heir to the throne of Iolcus. Following the death of his mother when he is still very young, his youth is heavily influenced by the presence of his father. He has a penchant for brutality and abuse of power. He is manipulative and self-aggrandizing.

He dominates his peer and fellow royal, Admetus, heir to the throne of the Thessalian fiefdom of Pherae. When Jason is unmasked as the son of Aeson, Acastus accepts with relish the challenge of undermining Jason's endeavours and seeking an opportunity to kill Jason and his friends, whilst ensuring Heracles can be enlisted into his expedition to Colchis. With the reputattion of Iolcus' power in Thessaly and his own bloodthirsty army, not to mention his manipulation of Mycenaean politics, he poses a signfiicant threat to Jason's plans, even if the hero can survive to the Argonaut voyage.