The 'Supergirl' Big Bad Revealed As SPOILER

Tonight's episode of Supergirl saw Kara discover that part of Krypton had survived, but viewers were in for another discovery that might have major implications for the heroes fight against Reign.

Spoilers for tonight's episode of Supergirl, "Dark Side of the Moon", below.

In the episode Kara (Melissa Benoist) and Mon-El (Chris Wood) go to an asteroid in deep space looking for a black Kryptonian rock that they believe will allow them to reverse engineer a cure for Sam/Reign (Odette Annable.) However, when they arrive they discover that the asteroid is actually Argo, a populous city-state that survived Krypton's demise thanks to a protective dome. They also discover that Kara's mother Alura Zor-El (Erica Durance) is alive and well and that the black rock they seek is what is keeping Argo and all its residents alive.

As taking even a small part of the rock has consequences for the well-being of the Kryptonian survivors, Alura brings Kara before a council where she must plead her case for why they should give of one thing keeping them alive to save Earth. One of the members of the council? A woman named Selena and fans of Supergirl have seen her before -- as the hologram that instructed and aided Reign in her Fortress of Sanctuary.

However, that's not the only thing significant about Selena. If you thought that "Dark Side of the Moon" felt a little bit like the 1984 Supergirl movie, you're right. The plots are a bit similar to one another, though inverted, and Selena was a part of that film, too. You see, in the movie, Supergirl comes to Earth in search of an immensely powerful item called the Omegahedron -- an item that, incidentally, was mentioned on Supergirl in Season One -- that Argo City needs in order to survive. However, the Omegahedron was found by Selena, a power-hungry witch who intends to use the device to take over the world. Her wicked plot is ultimately thwarted and Supergirl takes the Omegahedron back to Argo and saves the day.

On The CW series, things look like they may be a little less optimistic. After Kara tells the council that the rock is the only thing that will save Earth from Reign as the Worldkiller has gotten so strong, it's Selena that casts the tie-breaking vote to grant Kara's request. Kara and Mon-El take the rock and depart. However, alone after they leave Selena is seen donning the black cloak her hologram version wore and she calls out to Reign across the universe. Back at Lena's lab, Reign grows more powerful, destroying the force field holding her back just as Supergirl and Mon-El return, but with Selena alive, well, and still wielding some sort of power over Reign, they may need more than that black rock to stop the Worldkiller now.

Supergirl airs Mondays at 7/8c on The CW.