Achilles' heel

An Achilles’ heel is a deadly weakness in spite of overall strength, that can actually or potentially lead to downfall.

Mythological Origin
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In Greek mythology, when Achilles was a baby, it was foretold that he would die in battle from an arrow in the foot. To prevent his death, his mother Thetis took Achilles to the River Styx which was supposed to offer powers of invincibility and dipped his body into the water. But as Thetis held Achilles by the heel, his heel was not washed over by the water of the magical river. Achilles grew up to be a man of war who survived many great battles. But one day, a poisonous arrow shot at him was lodged in his heel, killing him shortly after. Still, Achilles is remembered as one of the greatest fighters who ever lived.