Achilles was the son of the Thetis, a nereid, and Peleus, the king of the Myrmidons. Zeus and Poseidon had been rivals for Thetis's hand in marriage until Prometheus, the fore-thinker, warned Zeus of a prophecy (originally uttered by Themis, goddess of divine law) that Thetis would bear a son greater than his father.For this reason, the two gods withdrew their pursuit, and had her wed Peleus.