This is ‘true’ in the same sense that Superman can only be stopped by Kryptonite. Achilles was the greatest of all the Greek heroes who besieged the city of Troy in Homer’s epic poem The Iliad. It is said that Achilles’ mother dipped him in the River Styx (the river of the Ancient Greek Underworld) to make him impervious to wounds. Achilles’ heel did not go into the water because that was where she was holding him and so it was the only place where he was vulnerable. In Greek mythology the Trojan prince Paris shot Achilles in the heel with an arrow and killed him. A person’s ‘Achilles’ heel’ has become proverbial for a critical weakness, like saying ‘the chink in the armour’. The city of Troy was later captured by the Greeks and its inhabitants killed or enslaved.
Answered by Bryan Sitch, Manchester Museum.