The Invulnerable Hero* is among our most treasured and recurring tropes.
It is the core feature of the great German epic, the Nibelungenlied. You probably know it better as the story of Siegfried from Wagner's treatment of the story in his famed Ring Cycle. Siegfried slays a dragon and bathes in its blood, that is, everywhere except for the spot on his back covered by a drifting linden leaf. He thus becomes invulnerable to harm except in this very spot.
To those of you more familiar with the Greek epics, you will no doubt see the parallels with the story of Achilles. Thetis takes an infant Achilles to the River Styx and dips him into its waters. He thus becomes invulnerable at every spot but the one covered by the fingers by which his mother held him beneath the Styx: his ankle.
For the more cultured among us, we have our Superman story. Our invulnerable hero with practically every possible advantage and a weakness to one substance that, like the Achilles heel, is so iconic an expression of the trope that it is now a euphemism for a singular weakness or point of failure. Kryptonite. And no, nerds and/or Ben, please do not email me your pedantic notes on red sun radiation, etc.