In the drawing above the Sea Goddess Thetis submerges her son Achilles in water from the Styx, to make him invulnerable. Unfortunately the water doesn’t touch the heel, where she holds him and his heel becomes his most vulnerable part.
When Achilles was a baby, it was foretold that he would die young. To prevent that his mother Thetis took Achilles to the River Styx, which was supposed to offer powers of invulnerability, and dipped his body into the water.
Achilles grew up to be a man of war who survived many great battles. One day, a poisonous arrow shot at him was lodged in his heel, killing him shortly afterwards.