Martha Kent, in desperation, holds Black Kryptonite up to her adoptive son, Clark Kent, to separate him from his Kryptonian side, Kal-El.
Black Kryptonite is a Varietal of Kryptonite, made by heating green Kryptonite to extreme temperatures. It can split Kryptonians into two separate beings; good and evil. The first appearance of black Kryptonite was in the Smallville TV series, but it was later introduced in the comics as a new form of Kryptonite.
When exposed to it, Doomsday was separated from his human identity of Davis Bloome, who killed Jimmy Olsen in Smallville season 8.
Both Superman and Supergirl have been exposed to black Kryptonite multiple times and have been forced to fight their evil self.
On Earth-22, the Batman Who Laughs exposes Superman, Supergirl and Jon Kent to a version of black Kryptonite he invented which causes them to turn violent and kill their families and each other.
Kara Zor-El/Supergirl, having been split into two people by Black Kryptonite.