In Greek mythology, Achilles’ sea-nymph mother was Thetis.
Thetis was a Nereid, one of the fifty daughters of the ancient sea god Nereus. Although she was immortal, Greek myths describe her as marrying the mortal king Peleus. Their wedding was attended by many gods and became the setting for Eris’s golden-apple dispute, which eventually helped lead to the Trojan War.
Thetis tried to make Achilles immortal in some versions by dipping him in the River Styx. The familiar story says she held him by the heel, leaving that spot vulnerable. This detail explains the later expression “Achilles’ heel,” although ancient sources do not all tell the story in exactly the same way.
Thetis also supported Achilles during the Trojan War and obtained divine armor for him from Hephaestus after Patroclus died. She is sometimes confused with Tethys, an older Titan associated with fresh water.