Thetis was a sea goddess and the goddess of water. She was also known as the leader of the fifty Nereides.
Thetis was the daughter of Nereus and Doris. Because of a prophecy that she was destined to bear a son greater than his father, Zeus had her marry a mortal man. Peleus, the chosen groom, was instructed to ambush her on the beach, and not release his grasp of the struggling goddess as she metamorphosed into a host of shapes. The couple were afterwards married in a ceremony attended by all the gods of heaven, and she bore a son, the hero Achilles.
In her desperate attempts to protect her son during the Trojan War, Thetis called in many favours from the gods. These included Hephaestus and Dionysus, both of whom she had given refuge in the sea as they faced crises of youth, and Zeus, whose throne she had protected by summoning the giant Briareus when the gods had sought to bind him.