Kryptonite

Kryptonite is a fictional material that appears primarily in Superman stories. In its best-known form, it is a green, crystalline material originating from Superman’s home world of Krypton that emits a peculiar radiation that weakens Superman, but is generally harmless to humans when exposed to it in the short term. There are other varieties of kryptonite such as red and gold kryptonite, which have different but still generally negative effects on Superman. Due to Superman’s popularity, kryptonite has become a byword for an extraordinary exploitable weakness, synonymous with “Achilles’ heel”. (Wikipedia)