SPOILER WARNING: This article contains major spoilers for "Dark Side of the Moon," the latest episode of Supergirl.
Supergirl just threw a wrench into Kara's plan to stop Reign. In "Dark Side of the Moon," the latest episode, she discovered Argo City, where her mother Allura and thousands of refugees have lived since the destruction of Krypton. Among them, Selina -- the Worldkiller priestess who created Reign -- enjoys a position on Allura's council.
Since Reign's emergence earlier this season, Selina has fostered and guided the Worldkillers. She has existed inside Reign's Fortress of Sanctuary like a hologram, in much the same way Jor-El does in Superman's Fortress of Solitude. As such, many viewers have assumed she -- like the rest of Krypon -- had died in the explosion that destroyed the planet and existed only as a program in the Fortress of Sanctuary.
However, Supergirl's trip to Argo City proved Selina is not only alive, but communicating with Reign directly -- a feat thought impossible. According to Allura, the dome protecting the city prevents the citizens of Argo City from communicating with the outside universe, but Selina's final scene proves that wrong.
What's worse, Selina is now privy to Supergirl's plan. Since Selina is a member of Allura's council, she was present when Supergirl asked for the Rock of Yuda Kal, the mythical rock that could separate Reign from Sam. Selina cast the crucial vote to allow Supergirl to do so, but that may bode ill for Earth, since the Rock can also be used to create a Worldkiller. At the end of the episode, Selina donned her black robe and reached out to Reign, likely to warn her of Supergirl's plan.
Airing Mondays at 8 p.m. on The CW, 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.