Achilles' Heel

In Greek myth, the sea nymph Thetis tried to make her infant son Achilles invincible by dipping him in the River Styx, the river of the underworld. She held him by one heel to dip him in, and that single spot, never touched by the water, stayed mortal. Achilles grew into the greatest warrior of the Trojan War, unstoppable until an arrow, some versions say fired by Paris and guided by the god Apollo, struck him in exactly that heel and killed him. It's worth knowing that Homer's Iliad, the oldest and most famous account of the war, never mentions this vulnerability at all; the heel detail first appears roughly a thousand years later, in the Roman poet Statius's Achilleid.