In the legend, Achilles was quickly dipped into the river Styx, held up only by the tendon of his heel. Touched by the dark sheath of water, every part of his body was instantly rendered impervious to even the most lethal weapon—except the undipped tendon. A simple arrow targeted to that vulnerable heel would eventually kill Achilles in the battlefields of Troy.
This is actually not true. Read the Illiad yourself.
