Kryptonite

Kryptonite is a fictional material from the Superman mythos — the ore form of a radioactive element from Superman's home planet of Krypton. It is famous for being the ultimate natural weakness of Superman and most other Kryptonians, and the word Kryptonite has since become synonymous with an Achilles' heel —the one weakness of an otherwise invulnerable hero.

Originating in the Superman radio show series, the material is usually shown as having been created from the radioactive remains of Superman's native planet Krypton, and generally has detrimental effects on Superman and other Kryptonians. The name "kryptonite" covers a variety of forms of the substance, but usually refers to the most common "green" form.