An Achilles’ heel is a fatal weakness in spite of overall strength, actually or potentially leading to downfall. While the mythological origin refers to a physical vulnerability, metaphorical references to other attributes or qualities that can lead to their downfall are common.
The death of Achilles was not mentioned in Homer's Iliad, but appeared in later Greek and Roman poetry and drama concerning events after the Iliad, later in the Trojan War. Here and in the myths surrounding the war, Achilles died from a heel wound as the result of an arrow - possibly poisoned - by Paris.
According to a myth arising later, his mother, Thetis, had dipped the infant Achilles in the river Styx, holding onto him by his heel, and he became invulnerable where the waters touched him -- that is, everywhere but the areas covered by her thumb and forefinger -- implying that only a heel wound could have been his downfall. Thetis was scared and fled out of fear. This possibly coincides with the story that Thetis left Achilles at a very young age.
The use of “Achilles’ heel” as an expression used for “area of weakness, vulnerable spot” dates only to 1855 (Merriam-Webster). It derived from the Greek "Achilleios pterna" (Greek: Αχίλλειος πτέρνα) literally meaning Achilles’ heel.
The anatomical basis of Achilles death is more likely to have been injury to his posterior tibial artery below the medial malleolus. It is noted that this area would have been included in Thetis’ grip and therefore would have been vulnerable.