What was Achilles' weak point?

In Greek mythology, Achilles is considered the greatest hero of the Trojan War. Son of the Nereid Thetis and Peleus, king of the Myrmidons of Thessaly, myth tells that as a child, he was immersed in the waters of the River Styx, which made him immortal. But since his mother held him by his heel, he was immersed in the waters, leaving his heel his only vulnerable spot. He is the most famous of Greek heroes.