Thetis the Nereid was fated to bear a son mightier than his sire. Zeus knew that, like his father Kronos and his grandfather Uranos, he was in danger from his own offspring, but he did not know who the mother of the formidable son was to be. Prometheus knew, for it had been revealed to him by his all-wise mother, Themis; but he was at feud with Zeus, and would not tell the secret, despite the torments inflicted upon him by the enraged god, and the promise made to him of release if he revealed it. Now both Zeus and Poseidon loved Thetis, and so a disaster to the Olympians seemed imminent. But the secret finally came out, being revealed either by Themis herself or by Prometheus, as the price of his freedom; and Thetis was wedded to Peleus, who being a mortal could not beget an immortal son.
There is another version, as old as the Epic Cycle, that it was Thetis herself who refused the attentions of Zeus, out of love and gratitude to Hera, who had brought her up. As Homer knows of the relations between Hera and Thetis, and nowhere mentions the prophecy of Themis (or Prometheus), it may well be that this version is very old, and the episodes now to be described a later addition.
But Peleus had, at least in the later versions of the story, first to catch her, which, as she was a sea-goddess, was not easy. His subsequent fortunes have all the flavour of a folktale. He had first of all, like Herakles with Nereus, to win his bride by wrestling, for she had no inclination to mate with anyone so inferior to her in rank. Contriving to meet her somewhere (according to Ovid, he caught her asleep on the shore), he at once assailed her; she tried to shake him off by turning into all manner of forms, as fire, a lion, a serpent, and so forth, but all to no purpose; so the marriage was duly celebrated, all the gods attending in great state.
But, as usual with a man who weds a fairy bride in the folklore of any country, his troubles were not at an end.
Thetis was determined to have an immortal child, and used an expedient very like that which Demeter employed with her nursling Demophon. Seven children were born, and one after another their mother threw them into fire, or according to others a boiling cauldron, either to test their powers or to burn away the mortal part that they had inherited from their father Peleus. At last Peleus interfered, and the baby Achilleus was either saved from destruction or prevented from becoming fully immortal. Thetis then, in high dudgeon, left her husband, exactly after the manner of fairy brides when crossed.
The rest of Thetis’ legend cannot well be separated from that of her son. It is to be noticed however that although the story of the burning or boiling of the children may be fairly early, it is not the earliest, for neither Homer nor Hesiod seems to have heard of it, or of any child of the marriage except Achilles.