Thetis
One of the 50 Nerieds (Beautiful sea nymphs); Mother of Achilles.
Thetis was the mother of Achilles, and, trying to make her newborn son immortal, dipped him into the river Styx (which had magical water), holding onto one of his ankles. The ankle did not touch the water, leaving it vulnerable.
Achilles later died in the Trojan War, having sustained an injury to his ankle.