Achilles was a hero of Trojan War and the main character in Homer's Epos, the "Iliad"

Achilles was the most handsome, capable and worthy of all the heroes that participated in the Trojan War. He was born in Farsala in southern Thessaly (Central Greece) as the son of Peleus, the King of Pithia in Thessaly, and the silver footed sea goddess Thetis.

Achilles' mother Thetis was strongly attached to her son and when he was born, she attempted to make him immortal by dipping him in the sacred waters of the river Styx. However, she did not quite achieve what she had hoped for... while she was keeping him inside the waters, she had to hold by one heel, leaving a vulnerable spot on Achilles' body. This weakness turned out to be crucial for Achilles, since he got killed during the Trojan War by an arrow that hit exactly that spot.

The expression "Achilles heel" remains until today, describing a small, but important weakness of a human being.