Black Kryptonite has the ability to split Superman into two different people his good and evil side. It also has the ability to split humans into a good and evil versions of one another.
Black Kryptonite: Black kryptonite was first introduced in the Smallville television series, in the fourth season premiere episode "Crusade," as kryptonite with the ability to split the personality of Kryptonians. It later appears in the fourth season episode "Onyx," where it is revealed to physically split the bodies of humans. In the series, black kryptonite can be created by super-heating green kryptonite. It later made its first appearance in a DC comic in September 2005's Supergirl #2, where it apparently possessed the ability to split a person or a person's personality into two separate entities. In Supergirl #3, Luthor used black kryptonite on Supergirl, which caused her to split into two separate people, one wearing Supergirl's traditional costume, and another wearing a black-and-white version. Her black-and-white costume is similar to the one that Superman was wearing when he returned from the dead. Luthor noted that he was given the black kryptonite by the god Darkseid, who may have been responsible for its creation. (A synthesized version of kryptonite in the feature film Superman III had similar effects on Superman.) In All-Star Superman #4, black kryptonite makes Superman evil, almost as if he is transforming into Bizarro Superman.