Kryptonite

Kryptonite is a radioactive element from Superman's home planet of Krypton. It is famous for being the natural weakness of Kryptonians.

The only sample of kryptonite originally found on Earth was a single fist-sized chunk that had been caught in the tail of the infant Kal-El's rocket and carried to Earth along with him upon the explosion of Krypton. This sample had been found by the scientist who resurrected John Corben as Metallo in a paranoid attempt to save the world from Superman. Metallo was soon kidnapped and his power source forcibly removed by Lex Luthor who, after discovering its debilitating effects on Kryptonians, created a ring with a kryptonite gem to keep Superman at bay. This backfired badly on Luthor, as long-term exposure to kryptonite radiation from the ring gave him cancer, leading to the amputation of his hand and then apparent death. Superman took possession of the ring and entrusted it to Batman, stating that he was the only person he could trust with the ability to kill him if necessary.

The amount of kryptonite on Earth increased dramatically after a meteor storm that accompanied the rocket that brought Kara Zor-El to Earth. Superman-friendly corporations, such as Wayne Enterprises and Kord Industries took it upon themselves to round up this influx of kryptonite, but much of it goes into illicit circulation or is stolen from holding facilities.

Involvement

 * Brainiac attempts to secure a catchment of kryptonite by attacking Project CADMUS's Area 51.

Villains

 * To defeat Supergirl as she attempts to free the roiting patients in the Metropolis General Hospital, Lex Luthor instructs the new villains to find a container of kryptonite he had moved to the hospital for just such an occasion.

Trivia

 * Kryptonite was first introduced in 1943 on the Superman radio series, as both a plot device and to allow Superman's actor, Bud Collyer, to take occasional time off. It was not until 1949 that kryptonite was incorporated into comic book mythos.
 * Kryptonite hurts Kryptonians as their cells absorb electromagnetic radiation from stars (like Earth's sun). Kryptonite's radioactivity interferes with this semi-photosynthetic process, driving the energy out of their cells in a painful fashion.