The fleece of the golden ram, which had saved Phrixus from being sacraficed.
Athamas, the king of Boeotia, had two children with his wife Nephele, a son Phrixus and a daughter Helle. Athamas then divorced Nephele, and married Ino, who also bore him two children.
Jealous of Semele's two children, Ino schemed to get rid of them. She convinced all of the women in the land to parch the grain set aside for the next year's crops. In the following year, when the grain would not grow, Athamas sent envoys to the Oracle at Delphi to ask what was wrong. Ino bribed the envoys to tell the king on their return that his son Phrixus must be sacrificed.
Athamas was then forced by his starving people to order the sacrifice of his son. Nephele, wishing to save her son, took Phrixus and Helle and sent them away on a golden ram, a gift to her from Hermes.
While the ram was crossing the strait between Greece and Asia, Helle lost her grip and fell to her death. It is from this that the Hellespont (the sea of Helle) getis it's name. Phrixus, on the other hand, arrived safely at Colchis and was greeted kindly by king Aeetes.
To repay Aeetes for his kindness, Phrixus sacrificed sacrificed the golden ram, and gave its golden fleece to the king. Aeetes nailed the fleece to an oak in a grove sacred to Ares, where it would be protected by a fierce dragon.