She was courted by both Zeus and Poseidon, but neither of them married her, out of fear of a prophecy that said Thetis' son would surpass his father in glory. Instead, she married Peleus, with whom she had a son, the mythical hero Achilles. Peleus' and Thetis' wedding was the main event that caused all subsequent events triggering the Trojan War. During the wedding, the goddess of discord Eris, who had not been invited, threw amidst the guests the Apple of Discord, on which the words "to the fairest" had been inscribed. Hera, Aphrodite and Athena started fighting over who should get the apple; Zeus said that Paris, prince of Troy should make the decision. He picked Aphrodite, after she told him she would give him the most beautiful woman in the world as a wife. This woman was Menelaus' wife, Helen.
When Hephaestus, the blacksmith god, was thrown from Olympus either by Hera or Zeus, it was Thetis along with Eurynome that helped him and placed him on the island of Lemnos. There, he worked for them as a blacksmith.
When Thetis gave birth to her son, Achilles, she decided to make him immortal by dipping him in the sacred waters of the river Styx, one of the rivers that flowed through the Underworld. However, when she was dipping him, she did not realise that his heel by which she held him was not touched by the waters, thus leaving that spot vulnerable. This is how the modern phrase "Achilles' heel" came to be; this was also the hero's doom, as an arrow shot by the prince of Troy, Paris, and guided by the god Apollo, hit Achilles' heel during the Trojan War, killing him. After his death, Thetis took her son's body and collected his ashes in an urn. She then started commemorative festivals in his honour.
Who was Thetis?
Thetis was a sea nymph in Greek mythology, or according to some myths, one of the Nereids, the fifty daughters of the sea god Nereus and Doris. She was courted by both Zeus and Poseidon, but neither of them married her, out of fear of a prophecy that said Thetis' son would surpass his father in glory.