An Achilles' heel is a fatal weakness in overall strength that can lead to one's downfall or death.
In Greek mythology, Achilles was the greatest Greek warrior in the Trojan War. He was the son of Peleus, king of Thessaly and the goddess Thetis. When he was a baby, his mother submerged him in the River Styx to make him immortal, all except for the heel that she held to dip him into the waters.
Achilles fought many battles during the 10-year Trojan War and finally killed Memnon, the king of the Ethiopians. As Achilles led the Greeks to the walls of Troy, he died from a heel wound caused by an arrow shot by Paris, the prince of Troy.