In Greek mythology, Achilles was a powerful warrior who had only one weakness: his heel, which was where his mother held him by her thumb and forefinger when she dipped him in the river Styx, thereby making him invulnerable everywhere the water touched him. Achilles eventually met his downfall when an arrow struck him in his sole weak area – his heel – and the term Achilles' Heel was born from the story. An Achilles' Heel is a sole weakness despite overall strength, generally leading to one’s downfall, and can take the form of a physical weakness or some other form of mental, logical, spiritual, moral, or otherwise metaphysical shortcoming.
The Achilles' Heel generally refers to one single weakness, a vulnerable spot in the midst of intense strength or general fortitude. This one trait is the only one that can lead to the person’s – or more often, character’s – downfall. While the Achilles' Heel has become somewhat cliché in film and writing – particularly in the world of superhero comics – its metaphorical implications hold true in many social situations. An Achilles' Heel has been aptly used in reference to wars, politicians, spiritual leaders, and countless other historical figures and events.
The Achilles' tendon connects the muscles of the calf with the heel bone in the foot. If severed, the injury would most likely not be fatal, though it would be painful and debilitating. In the case of Achilles, the arrow he had been struck by was most likely poisoned, and since it hit his only invulnerable spot, it led to his death. As a means of Deus ex Machina, Achilles’ Achilles' Heel was, quite literally, his heel. In other instances throughout the literary world, an Achilles' Heel does not necessarily take on the role of physical limitation, nor does it always lead to the character’s death; many characters meet their end through extreme greed, thereby finding themselves impoverished by the end of the story despite their great wealth at the start of the story. Their Achilles' Heel, in this regard, is their pursuit of wealth, which did not lead to their death but instead to their ruination.
A very recent literary example of an Achilles' Heel occurs in the Harry Potter series of books, written by J.K. Rowling. Voldemort, the primary villain in the series, seems all-powerful and generally more gifted as a wizard than any other character in the books. His one weakness, however, is love: his Achilles' Heel is his inability to love and form friendships. Using this weakness, Harry Potter eventually defeats the powerful wizard and Voldemort succumbs to his Achilles' Heel.