Metamorphoses Book 11: Peleus and Thetis
Jove avoided the bed of the sea-nymph, Thetis, because Proteus foresaw that she would bear a son who would surpass his father's greatness. Jove didn't want to be overthrown. Instead, he encouraged Peleus, his grandson, to take Thetis as a wife, and so Peleus tried to rape her while she lay on the beach. She kept changing shape so that he couldn't quite capture her, and he had to give up. Peleus prayed to the sea gods for help, and Proteus told him to tie Thetis down so that no matter what form she took, she'd not be able to escape. When he tried it the next day, Thetis gave up because she knew he'd gotten help from one of the gods. They were married, and Achilles was the product of their union.