Thetis (thēˈtĭs) [key], in Greek mythology, a nereid, mother of Achilles. She was loved by both Zeus and Poseidon, but because of a prophecy that her son would be greater than his father, the gods gave her in marriage to a mortal, Peleus. According to one legend, Thetis burned alive her first six sons and sent their immortal spirits to Olympus. Peleus, however, snatched the seventh, Achilles, from the fire and sent him to be raised by the centaur Chiron. See Paris, in Greek mythology.
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