In Greek mythology, Achilles was the demi-god and hero of the Trojan War who was invulnerable in every part of his body, except for his heel which was left unprotected after his mother Thetis tried to make him immortal by dipping him into the river Styx, and accidentally left his heel as the only part of his body not soaked in the river's water.
Achilles was ultimately brought down by an arrow, shot by Paris into his heel.
Implied to bring down immortals.
This artifact is collected in the Warehouse in the aisle Corfu-783. Artie did some digging and thought that the arrow that brought down Achilles would be able to strike the immortal Paracelsus.