Jason and the Argonauts

Pelias (Douglas Wilmer), misinterpreting the prophecy given to him by the god Zeus (Niall MacGinnis), usurps the throne of Thessaly, killing King Aristo and most of his family. The infant Jason is spirited away by one of Aristo's soldiers. Pelias slays one of the king's daughters, Briseis (Davina Taylor), as she seeks sanctuary in the temple of the goddess Hera (Honor Blackman). Because the murder has profaned her temple, the angry Hera becomes Jason's protector. She warns Pelias to beware "of a man wearing one sandal".
Twenty years later, Jason (Todd Armstrong) saves Pelias from drowning (orchestrated by Hera), but loses his sandal in the river; Pelias recognizes him from the prophecy. Learning that Jason intends to find the legendary Golden Fleece, he encourages him, hoping Jason will be killed in the attempt.
Jason is brought to Mount Olympus to speak with Zeus and Hera. Hera tells him Zeus has decreed he can only call upon her for aid five times. She directs him to search for the Fleece in the land of Colchis. Zeus offers his direct aid, but Jason declares he can organize the voyage, build a ship, and collect a crew of the bravest men in all Greece.
Men from all over Greece compete for the honor. Because their ship is named the Argo after her builder, Argus (Laurence Naismith), the crew are dubbed the Argonauts. Among them are Hercules (Nigel Green), Hylas (John Cairney), and Acastus (Gary Raymond), the son of Pelias, sent by his father to sabotage the voyage.
Hera guides Jason to the Isle of Bronze, but warns him to take nothing but provisions. However, Hercules steals a brooch pin the size of a javelin from a treasure building, surmounted by a gigantic statue of Talos, which comes to life and attacks the Argonauts. Jason again turns to Hera, who tells him to open a large plug on Talos' heel, to release the giant's ichor. Talos falls to the ground, crushing Hylas, hiding his body. Hercules refuses to leave until he ascertains the fate of his friend. The other Argonauts refuse to abandon Hercules, so Jason calls upon Hera again. She informs them that Hylas is dead and that Hercules will not continue on with them.
The Argonauts next reach the realm of King Phineus, who has been blinded and is tormented by harpies for his transgressions against the gods. In return for his advice on how to reach Colchis, the Argonauts render the harpies harmless by caging them, whereupon Phineus tells them to sail between the Clashing Rocks, which destroy any ship in the narrow channel, and gives Jason an amulet. Arriving at the Clashing Rocks, the Argonauts witness another ship suffering that fate. When the Argo tries to row through, the ship appears doomed. Jason throws Phineus' amulet into the water, and the sea god Triton rises up and holds the rocks apart so the Argo could pass them. The Argonauts rescue a survivor from the other ship, Medea (Nancy Kovack), high priestess of Colchis.
Challenging Jason's authority, Acastus engages him in a duel. Disarmed, Acastus jumps into the sea and disappears. Jason and his men land in Colchis and accept an invitation from King AeÃ«tes (Jack Gwillim) to a feast. Unknown to them, Acastus has survived and warned AeÃ«tes of Jason's quest for their prized Golden Fleece. AeÃ«tes has the unwary Argonauts imprisoned, but Medea, having fallen in love with Jason, helps him and his men escape.
Meanwhile, Acastus tries to steal the Fleece, but is killed by its guardian, the Hydra. Following right behind Acastus, Jason is able to kill the beast and retrieve the gift of the gods. AeÃ«tes, in pursuit, sows the Hydra's teeth while praying to the goddess Hecate, producing a band of skeletal warriors. Jason, together with Phalerus and Castor, hold off the skeletons while Medea and Argus escape back to the Argo with the Fleece. After a prolonged battle in which his companions are killed, Jason jumps from a cliff into the sea,[2] and he, Medea, and the surviving Argonauts begin their voyage home to Thessaly. In Olympus, Zeus tells Hera that in due time he will call upon Jason again.

Question: What was Acastus trying to steal?
Answer: The Fleece