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, made up of that person's good side and his or her evil side. Its first appearance was in the Smallville TV series as an original concept, but eventually made its way into the comics as a new form of Kryptonite.

When exposed to it, Doomsday was separated from his human identity of Davis Bloome, who still committed a murder out of jealousy, killing Jimmy Olsen in Smallville, season 8.

Both Superman and Supergirl have been exposed to the rock at various times, and have been forced to fight their evil side.

In the Batman Who Laughs' universe of 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 into violent savages and makes them kill their families and each other.

Kara Zor-El/Supergirl, having been split into two people by Black Kryptonite.

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