What is Black Kryptonite, The Substance Lex Luthor Wanted on 'Supergirl?'

In last night's episode of Supergirl, Lex Luthor wanted to get his hands on some Black Kryptonite. While it was all wrapped around a story about finding a miracle cure for his (and James Olsen's) ailments, there is a pretty good chance that he had an ulterior motive for his interest in the substance, which made its debut on Smallville and has gone on to appear in several comic book stories, as well as on Justice League Action, the all-ages-friendly animated series from Cartoon Network. The basic idea? It breaks up a being (usually a Kryptonian, although sometimes a human) into two distinct halves -- usually their "good" and "bad" sides.

If that sounds like a potentially alarming thing for Lex to be looking for, in the run-up to his appointment next week with the Red Daughter Supergirl, that just means you've been paying attention all this time. It is difficult to guess how Lex would benefit from splitting up Supergirl's (or the Red Daughter's) personality, but it is equally difficult to believe that a plot element like this would be introduced so soon after the revelation of a second, presumably evil Supergirl and that the two would be totally unconnected.

Here's a rundown of Black-K's properties, from the DC Wiki:

• First seen on TV's Smallville, the only time it was ever used on Clark was in the Season Four episode "Crusade", when he was reprogrammed as Kal-El. Martha Kent placed a chunk of Black Kryptonite (given to her by Dr. Swann's assistant, Bridgette Crosby) on his chest and the two personalities physically split. Clark then used the Black Kryptonite to eliminate Kal-El.
• In the Season Eight episode "Doomsday" Chloe used Black Kryptonite to split Doomsday from Davis Bloome turning them into two different entities.
• During certain circumstances, perhaps only at the time of creation, Black Kryptonite can also affect humans, causing them to split into two different beings, one manifesting a good and pure personality, and the other an evil personality. The only case of it ever affecting humans occurred in the episode "Onyx" when an experiment with Green Kryptonite went awry and caused Lex Luthor to split in two.
• In Post-Crisis on Infinite Earths continuity, Black Kryptonite was used by Lex Luthor at the behest of Darkseid to expose Supergirl's true "nature". It had one of two effects, on Kara Zor-El it split her negative persona from her pure self as it does in Smallville. While in Kal-El's case he said it stripped him of his sanity, turning him evil for an undisclosed time.
• In All-Star Superman, Black-K has the effect of inverting the moral standard of the individual Kryptonian affected by it, turning Superman bad for a short time while also causing his abilities to weaken gradually alongside his mental acuity.
• In Justice League Action, it has similar persona separating effects, splitting Superman's good and evil sides, which wear a white and a black suit respectively.
• The Batman Who Laughs used a modified version of it on his universe's Clark and Jon Kent to drive them into a killing frenzy, ripping apart their universe's Lois Lane and each other.

What do you think Lex might be up to in his quest for Black Kryptonite and his mysterious connection to Red Daughter? Sign off in the comments. Supergirl airs on Sunday nights at 8 p.m. ET/PT, before episodes of Charmed on The CW.