the.com/achilles heel

means A single vulnerability or flaw that can bring down an otherwise strong person, plan, or thing.

from From the Greek myth of Achilles, whose mother dipped him in the River Styx to make him invulnerable — but held him by the heel, leaving that one spot untouched by the magic water. He survived countless battles only to die from an arrow to that very heel. The phrase itself as a fixed English idiom is a much later coinage, popularized in the 19th century, long after the myth itself was ancient.