Sea nymph who was the mother of Achilles with her mortal Peleus husband. Her father was the sea god Nereus and mother Doris.
Peleus had conquered Thetis by holding on to her while she was taking on different forms - everything from animals to fire. According to another version the gods had ordered her to marry a mortal.
Both Zeus and Poseidon had wanted to marry her, but when Themis prophecised that her son would be stronger than them both, they considered it wiser to give her to a mortal. It was at their wedding the Trojan war became an issue. They had invited all the gods except the godess of arguments and misunderstandings, Eris, and she took her revenge by throwing a golden apple with the inscription "to the most beautiful" into the party, resulting in the argument between Hera, Athena and Aphrodite.
Thetis made Achilles immortal by holding him as a baby over a fire or into the river Styx. Because she held him by his heel, that was the only mortal part of him. She would often help Achilles with advice and warnings.