Dark Supergirl's History
This evil version of Supergirl was culled from the recesses of Supergirl's subconscious mind with Black Kryptonite.
Lex Luthor had acquired an upgraded version of his battlesuit from Darkseid. Forged in the Fire Pits of Apokolips, this particular armor was weaponized with multi-colored Kryptonite energy projectors housed within its gauntlets. Luthor knew that by using a Black Kryptonite energy ray, he could possibly corrupt Supergirl, bending her will to his own.
Luthor and Supergirl fought against each other in Monument Valley, Utah. He exposed Supergirl to the Black Kryptonite ray, but rather than corrupting her as he had expected, the ray actually generated a second, darker Supergirl.
This evil Supergirl possessed all of the normal Supergirl's memories and powers. As it turned out however, Supergirl's own memories were warped due to prolonged Kryptonite poisoning, and as such, her doppelgänger suffered from the same inaccurate, maladjusted memories. She believed that Supergirl's father, Zor-El, had sent her to Earth on a secret mission to kill her cousin, Superman. The evil Supergirl, completely embracing her Kryptonian heritage, sought to fulfill this psychic programming and was not inhibited by her counterpart's strong moral psyche.
The evil Supergirl fought against Lex Luthor, ultimately flying him to the moon and smashing him through the walls of the Justice League Watchtower. At this point, she encountered the Justice League and was able to take each of them down one by one. Using super-speed, she snatched John Stewart's Green Lantern Ring from his finger before he could use it against her, then slammed him into a wall. She made short work of both Black Canary and Hawkman. The Flash attempted to subdue her by wrapping her up with a length of strong cable, but Supergirl zoomed off across the surface of the moon and the Flash gave chase. The Flash realized only too late that he could not withstand the lack of breathable atmosphere on the moon, and Supergirl was able to take him down. By this point in the battle, Lex Luthor had found a way to escape her clutches.
The real Supergirl, having recovered from Luthor's attack, tracked her double to the moon and engaged her in combat. Their battle ultimately brought them back to Earth and to the waterfront district of Gotham City. While the two Supergirls fought with one another, the heavy hitters of the JLA, Batman, Superman and Wonder Woman arrived to reconcile the situation. The evil Supergirl sowed confusion by switching costumes with her progenitor at super-speed. Now, nobody could tell who was the real Supergirl and who was the evil one. Batman was able to keep them both at bay with the help of his Kryptonite Ring, and Wonder Woman managed to bind them both together with the Lasso of Truth. Demanding to know who the real Kara Zor-El was, the mystical power of the lasso merged both Supergirls back together and the evil Supergirl ceased to exist as a singular entity.
A long while later, during a Justice League mission, Supergirl was subjected to Omega Man's power and turned into Dark Supergirl. However, that version of Dark Supergirl was bratty, self-centered and obnoxious, but not really evil. Kara appeared to use that persona like a mask to run away from her past trauma.
Doctor Fate diagnosed Supergirl suffered from a severe case of survivor guilt and PTSD, and "Dark Supergirl" was a physical manifestation from her psychological trauma, which was literally crushing her. Then Nelson tells if Kara wants to survive, she has to forgive herself and accept that she is blameless for the destruction of Krypton and New Krypton and the deaths of her parents.
Kara followed his advice, and her "Dark Supergirl" identity gradually faded once again.
