[61] His mother, Thetis, the daughter of Nereus and Doris, who was courted by Neptune and Jupiter. When, however, it was known that the son to whom she would give birth must prove greater than his father, it was determined to wed her to a mortal, and Peleus, with great difficulty, succeeded in obtaining her hand, as she eluded him by assuming various forms. Her children were all destroyed by fire through her attempts to see whether they were immortal, and Achilles would have shared the same fate had not his father rescued him. She afterwards rendered him invulnerable by plunging him into the waters of the Styx, with the exception of that part of the heel by which she held him. Hygin. Fab. 54