Supergirl is on hiatus until April, but before going on break The CW superhero series dropped a little bit of a bomb. Not only did we find out that Imra (Amy Jackson) and Mon-El's (Chris Wood) marriage was an arranged one, but that the Legion of Super-Heroes are in National City as part of a secret mission -- one that not even Mon-El knew about.
As the show ended on that cliffhanger, fans have been left to theorize what that secret mission really is. While there are nearly limitless possibilities for the secret mission many think that the real reason the Legion has travelled to 21st century National City isn't that they accidentally ended up there but that they are really there to save Supergirl herself. To explain it, one has to consider some of the information that Mon-El the Legionnaires have already shared with Supergirl and the DEO. For starters, Mon-El tells Team Supergirl that there is a massive gap in Earth's history occurring in the 25th century when an unknown cataclysm/extinction level event wipes out much of Earth's population, decimates its culture, and leaves behind precious few records. From before that event, what is known is that Supergirl recovers from her fight with Reign and that, sometime in the future, Reign simply leaves.
We also find out in that same episode that a thousand years into the future a catastrophic event impacts the planet Winath leading to a once-thriving planet to become decimated by something called The Blight. The Legion figures out how to stop The Blight, codes the information into their DNA, and then plans to preserve it to go back to the point in the future where they can stop The Blight. It's possible that the two situations, the cataclysm in the 25th century and the The Blight catastrophe on Winath are connected and both of them tie back to Supergirl's fight against Reign.
While the specifics of the 25th century disaster aren't clear, it's possible that whatever wipes out Earth's population and culture could be similar to The Blight on Winath. The Blight are described as consuming anything and everything in their path. Compare that to the name of the third, yet-to-be-revealed Worldkiller on Supergirl. In the last episode, Purity awakened meaning that only Pestilence is left. Pestilence generally refers to and is associated with infectious disease and plague in animals. Blight has the same associations in the world of plants. It's possible that the mysterious Blight that destroys Winath is the Worldkiller Pestilence herself or another, related Worldkiller. If so, that might shed some light on the historical note that Reign simply "left".
The possible connection between Blight and Pestilence plays out in the theory that yes, Supergirl survives her initial fight with Reign, but is killed fighting Reign along with Purity and Pestilence. With Supergirl gone, Reign is able to take the next steps of her plan with Earth and at some point, departs along with the other Worldkillers only to return later on, this time with no one to interfere and stop them, leading to the catastrophic event and gap in records the Legionnaires referenced. This would free the Worldkillers, particularly Pestilence, to go on and destroy other worlds -- including Winath.
If the devastation of Winath and the future devastation of Earth are connected to the Worldkillers and that Supergirl's defeat is what lead to it, the Legion might really be in the present to aid Supergirl and help her defeat the three Worldkillers so as to prevent future events. This would explain, to an extent, why Imra was so eager to get involved with fighting Reign while Supergirl was in a coma. It would also explain why Imra and Brainy both seemed less concerned about protecting their DNA-encoded information about the Blight than Mon-El was. Mon-El, kept in the dark about the real mission due to his feelings for Kara -- thinks the information needs to be preserved while Imra and Brainy know that they are going to need to use it in the very near future to save the universe itself by saving the Girl of Steel.
Supergirl airs Mondays at 8/7c on The CW. The show is now on hiatus until April 16th.