So that the prophecy is not fulfilled, the king of the gods hurried to marry the beloved with the mortal Peleus, king of Phytia (Thessaly), son of Eacol and grandson of Zeus, on the part of father, and great friend of Heracles. Tethys, however, fled the court of the groom, transforming himself into several elements. On the advice of the centaur Chiron, Peleus held it violently, until the nereid returned to its natural form. The wedding was celebrated in the presence of the gods and the muses. Seven children were born from the union. To purify the children of the mortal elements inherited from their father, Tethys exposed them to fire, resulting in his death.
According to one tradition, when he tried to purify his seventh son, Achilles, Peleus intervened, saving the child. Annoyed, Tethys left her husband and returned to the bottom of the sea. She protected her son throughout the hero's life, trying to keep him from danger and comforting him in sadness. She was unable, however, to prevent him from dying in the Trojan War, as she had thus decreed Fate. After the hero's death, she took Neoptolemus under her protection.