Supergirl / DC’s Legends of Tomorrow AU: In the year 2017, Kara Zor-El is lured to save her sister from an evil secret organization. The kidnapping is a ploy to unleash red kryptonite on Kara and have her destroy National City. However, when Kara accidentally kills Alex, the darkness inside her grows exponentially until she destroys all Earths. Rip Hunter sets a course to the future of Kara’s Earth and sends Sara to deal with Supergirl, both because she understands being devoured by darkness and because she relates to losing a big sister. Instead of letting Alex die, though, Sara decides to go save her in Kara’s place.
Ending scene cliffhanger for next season of this thing: