Proteus, an early sea-god, advised Peleus to
find the sea nymph when she was asleep and bind her tightly to keep
her from escaping by changing forms. She did shift shapes, becoming
flame, water, a raging lioness, and a serpent. Peleus held fast.
Subdued, she then consented to marry him. Thetis is the mother of
Achilles by Peleus, who became king of the Myrmidons.