What Is Supergirl's Black Kryptonite, and What Does It Do?

The CW's Supergirl has introduced a number of threats in its fourth season, including Sam Witwer's Agent Liberty, the head of an anti-alien campaign targeting Kara and her allies. Viewers have also seen the live-action debut of Manchester Black, continuing the show's incorporation of lesser-known Superman villains into the Girl of Steel's rogues gallery.

This season also marked the long-awaited arrival of Superman's archenemy, Lex Luthor (played by Jon Cry) -- and when he's involved, you can almost guarantee an appearance by Kryptonite, in some form. We've already seen two kinds of Kryptonite on Supergirl: Red Kryptonite, which turned Kara evil; and, most recently, the more common Green Kryptonite, which was infused into the atmosphere, forcing Kara to wear a protective suit. The most recent example of Luthor's connection to Kryptonite occurred on the latest episode of Supergirl, in which he attempted to cure his and James Olsen's malady using the Harun-El, or Black Kryptonite, which has appeared a few times on Supergirl, beginning with the third season's "The Fanatic."

The Harun-El has exhibited different properties with each appearance, including being able to create the dangerous Worldkillers while also splitting Sam from her Reign persona.

In the Season 3 finale "Battles Lost and Won," the substance was responsible for the appearance of a duplicate Supergirl in Siberia, who showed up again in the fourth season, teasing the adaptation of the Red Daughter version of Supergirl from Superman: Red Son. So what is Black Kryptonite and what does it to do to Supergirl, and other characters from Krypton?

The various types of Kryptonite are remnants of the Krypton that were irradiated by the planet's destruction, making the substance dangerous to Kryptonians like Supergirl and Superman. Kryptonite first appeared on the Adventures of Superman radio serial before transitioning to the comics in 1949.

The first, and most common, form is Green Kryptonite, which weakens, and can eventually kill, Kryptonians. Kryptonite of various colors has been introduced over the years, with each hue producing a different reaction.

The Kryptonite spectrum received a dark addition in Smallville's Season 4 premiere, "Crusade," in which Clark Kent's identity was overwritten with the Kryptonian Kal-El personality by his father, Jor-El. Black Kryptonite was introduced and used to split the personalities of Clark Kent and Kal-El, and then merge them again, with Clark in control. The substance was utilized again in Season 8 to split Davis Bloom from the monster Doomsday, cementing the primary ability of Black Kryptonite. This mimics the attributes of synthetic Kryptonite first seen in Superman III, in which the Man of Steel turned evil before splintering into a fully evil Superman and the righteous Clark Kent.

In the Smallville universe, Black Kryptonite is created by super-heating Green Kryptonite, although it has had different origins in the comics. It actually first appeared in the comics in Supergirl Vol. 5 #5, making its reappearance on The CW's Supergirl both a continuation of its cinematic origins and an homage to the source material.

The fifth volume of Supergirl, by Jeph Loeb and Ian Churchill, featured the first version of Kara Zor-El, cousin of Superman, since DC's Crisis on Infinite Earths. During the opening arc, Supergirl is exposed to Black Kryptonite by Lex Luthor, which results in the separation of her into two entities. Good Supergirl retained her normal costume, while Bad Supergirl wore a black-and-white version. Eventually, the two halves were rejoined, although the dark half of Supergirl continued to influence her throughout the series.

That first comic appearance of Black K may provid insight into how producers intend to adapt their own version of the Red Daughter for Supergirl.

With the appearance of the duplicate Supergirl in Siberia, the stage has been set for a Russian/Red Son version of Supergirl, although without the alternate-reality aspect that The Flash has utilized extensively. Supergirl's use of Black Kryptonite has allowed the series to further explore the unique abilities of the Smallville-created substance while also bringing in a fan-favorite alternate version of Kara. And that could be only the beginning.

Luthor's plans for the Black Kryptonite undoubtedly go beyond curing himself (and, by extension, Olsen), and we only have to look toward the comics again to see some of the dangers that come from tampering with the substance. In Dark Nights Metal: The Batman Who Laughs #1, the Joker-corrupted Batman modifies Black Kryptonite, which forces Supergirl to rip apart her family before dying, with Superman and Superboy similarly killing Lois Lane before turning on each other when exposed to the modified Black K.

Supergirl has already established other versions of the Black K, with Lena Luthor synthesizing her own sample of the Harun-El, which Lex could easily further manipulate for his own purposes. While the Black K created-Red Daughter version of Supergirl is definitely a threat to watch out for in the coming weeks, Lex Luthor's plans could be infinitely more dangerous.

Airing Sundays at 8 p.m. ET/PT on The CW, Supergirl stars Melissa Benoist as Kara Danvers, David Harewood as Martian Manhunter, Mehcad Brooks as Jimmy Olsen, Chyler Leigh as Alex Danvers, Katie McGrath as Lena Luthor, Jesse Rath as Brainiac-5, Nicole Maines as Nia Nal and Jon Cryer as Lex Luthor.

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Supergirl's Black Kryptonite: What Is It, and What Does It Do?