kryptonite

Kryptonite is a fictional material that appears primarily in Superman stories published by DC Comics. In its best-known form, it is a green, crystalline material originating from Superman's home world of Krypton that emits a unique, poisonous radiation that can weaken and even kill Kryptonians. Kryptonite radiation can be transmitted through any element except lead. There are other varieties, such as red kryptonite and gold kryptonite, which have different but still generally negative effects.

Adversaries of Superman and other characters are frequently depicted using kryptonite against Superman, with Lex Luthor incorporating it into weapons, Metallo being powered by it, and Titano able to project its radiation from his eyes. Kryptonite has become a byword for an extraordinary exploitable weakness, synonymous with "Achilles' heel".