Thetis was one of the Nereids (sea nymphs) in Greek Mythology. She was the mother of the hero of the Trojan War, Achilles. She made Achilles immortal when he was born, by dipping him in the River Styx which flowed through the Underworld. However the heel with which she held the baby Achilles was not dipped in the water, which ultimately became his doom. He was struck by and arrow through his heel from the bow of Paris during the war, as revenge for his killing of Paris' brother - Trojan Prince Hector.