In Greek mythology , a Nereid loved by both Zeus and Poseidon .
When it was revealed that Thetis was destined to bear a son who would be mightier than his father, the two gods gave her to Peleus, king of the Myrmidons. Unwilling to wed a mortal, Thetis resisted Peleus' advances by changing into various shapes, but Peleus finally captured and married her. Their child was Achilles . Some legends relate that she bore seven children, all of whom perished either when she tried to render them immortal or when she killed them as the offspring of a forced marriage.