Achilles

Thetis Dipping the Infant Achilles into the River Styx by Peter Paul Rubens (c. 1625; Museum Boijmans Van Beuningen, Rotterdam)

 

Thetis was a sea nymph and Achilles was her son.  She knew of the destiny of her son and so at birth she dipped him into the water of the river Styx  to protect Because sh eheld him by the heel that part of his body was unprotected and vulnerable.