In an effort to make her son immortal, according to Statius, Achilles' mother, the nymph Thetis, held her son by the ankle while dipping him into the River Styx. This made Achilles invulnerable, except where Thetis' fingers gripped him. Since his vulnerable spot was his heel, the term Achilles' heel is used for someone's weak spot.
Achilles was injured, so his invulnerability wasn't total, but then, the immortal goddess Aphrodite was injured (by Diomedes), too.
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