Thetis is a Nereid, the daughters of Nereus and Doris. Prometheus had prophesized that her son would overthrow Zeus, and so Zeus wanted nothing to do with her, although she was very attractive. To solve the problem, Zeus married her to the hero Peleus. This way, she could only bear mortal children, who could not overthrow Zeus in Olympus. Ovid writes that Peleus first had to capture Thetis, who constantly changed shapes in an attempt to avoid him. It was the wedding of Thetis and Peleus that caused the Trojan War, because of the golden apple.
After the wedding, Thetis began to bear children with her husband Peleus. Still wishing to have an immortal child to overthrow Zeus, Thetis tested the immortality of her children. She did this by placing each of her newborn children upon a fire. Every time, the child died. After several attempts, she deemed a different a different stratagem necessary. This son she would make invulnerable; the next best thing to being immortal. She took her final son, Achilles to the underworld and dipped him into the river Styx. As she was about to fully submerse her child, she was startled by Peleus. He must have feared losing yet another son, and followed his wife to the underworld. Because of this, Thetis did not completely dip son in the river, leaving the young Achilles' heel still vulnerable. Enraged, Thetis abandoned her husband and child, returning to original home, the sea.
Thetis did not stop helping her family, however. On his voyage with the Argonauts, Peleus was aided by his former wife Thetis. She took especial interest in the life of her son Achilles. She knew that her son was fated to die in war against the Trojans, so she attempted to keep him out of the Trojan War. She took him to the court of Lycomedes, on the island of Scyrus. She persuaded Lycomedes to take her son in, and to raise him as a girl. Odysseus, was not fooled by this ruse, however, and Achilles, with his fifty Myrmidian ships, accompanied Odysseus to Troy. During the tenth year of the war, when Agamemnon forced Achilles to give up the girl Briseis, Thetis went to Zeus for help. In order that her son should receive more glory, she persuaded Zeus to cause the Trojans to win, until the Greeks begged Achillesto return to the battle. After the death of his friend, Patroclus, he eventually did return to battle. To aide him during the fight, Thetis gave her son armor crafted by Hephaestus himself. After the death of Hector, Thetis counseled Achilles to finally give up the body.