Thetis was a very beautiful sea nymph. Thetis was prophesied to have a son that would become more powerful than his father. Once this had spread through Olympus she was no longer desirable, despite her stunning beauty. Zeus had even tried to court her but ceased because of this prediction. At last Thetis married the mortal Peleus and gave birth to the mighty warrior Achilles.

Since Achilles was the son of a mortal, Thetis tried to make him immortal by dipping him into the River Styx. She held him by his heel and thus his heel became the only mortal part of his body. This is where we get the phrase "Achilles' Heel" today. Achilles was only defeated in the Trojan War when he was hit in his heel by a poison tipped arrow.