SPOILER WARNING: This article contains major spoilers for “Battles Won and Lost,” the Season 3 finale of Supergirl.
Supergirl's new villain wears a very familiar face: her own. In "Battles Won and Lost," the Season 3 finale, Kara used the Rock of Yuda Kal to help Sam Arias defeat her alter ego Reign. However, the Rock's use led to an unintended consequence: the birth of Dark Supergirl.
During the final showdown between Team Supergirl and Reign, the Worldkiller managed to get Kara in a headlock. In response, Kara called for Mon-El to toss her the Rock of Yuda Kal. She pressed it into Reign's hand and called for Sam to join them. With all three hands on the Rock, they were transported into their subconscious, where Sam was able to put an end to Reign once and for all.
Although Team Supergirl was ultimately successful in their mission, they didn't realize the Rock had split a part of Kara away from herself, in much the same way they used it to split Reign from Sam. The tag at the end of the episode jumped from Kara's studio to Siberia, where a group of Russian soldiers were shocked to discover a figure walking towards them through a snow storm. When the figure came into focus, it was Supergirl, naked but for the black cloak wrapped around her shoulders. The fact that the Rock of Yuda Kal created this Supergirl doppelganger implies that it may secretly be Black Kryptonite.
In the comics, a dark version of Supergirl debuted in 2005's Supergirl #3 when the Girl of Steel was subjected to a ray of Black Kryptonite. Instead of corrupting Supergirl, it created a doppelganger of her based on the original's subconscious. This dark version of Supergirl -- who wore a black costume -- inherited warped memories, leading her to believe that her father had sent her to Earth to kill Superman. With a little help from Wonder Woman's Lasso of Truth, Kara was ultimately able to reabsorb her evil twin. Doctor Fate believed Dark Supergirl was a manifestation of Kara's survivor guilt and PTSD.
This won't be the first time The CW's Supergirl fought off an evil doppelganger. In "Crisis on Earth-X," this past season's four-show Arrowverse crossover, Kara went toe-to-toe with a version of herself called Overgirl, who hailed from Earth-X. Raised in a world where the Nazis won World War II, Overgirl hoped to conquer the multiverse with her husband Oliver Queen, aka Dark Archer. Overgirl eventually exploded due to solar poisoning.
Returning to The CW this fall, Supergirl stars Melissa Benoist as the Girl of Steel, David Harewood as Martian Manhunter, Mehcad Brooks as Jimmy Olsen, Chyler Leigh as Alex Danvers and Jeremy Jordan as Winn Schott, with appearances by Calista Flockhart as Cat Grant and Tyler Hoechlin as Superman.