I would say that the answer is his strength.
During the Trojan War, Achilles was a great warrior known for slaying Hector outside the gates of Troy. His resilience was one of his greatest strengths as he was dipped in the waters of the River Styx in the Underworld as a child, granting him invulnerability all over his body. However, he had one weakness: his heel. His mother held onto his heel while dipping him in the River Styx, and thus, this is the only place on his entire body that could kill him. Achilles got overconfident as a result of his immunity, and he was caught off guard when Paris presumably shot him in the heel with an arrow, killing the great warrior.
