Warning: spoilers ahead for Episode 22 of Supergirl Season 3, called "Make It Reign."
Only one episode is left in the third season of Supergirl, and the penultimate installment set the stage for a catastrophic finale that threatens to destroy the world as the humans of Earth-38 know it. Reign isn't quite as dead as Kara and the good guys thought after last week's episode, and the rogue Kryptonians (led by Selena) found a way to bring her back with the blood of the Worldkillers. They invaded the D.E.O., killed one of the loyal agents, and wreaked all kinds of havoc. Despite Kara and Co.'s best efforts, the Kryptonians brought back Reign and kicked off their plan to terraform Earth to be more like Krypton, which means the eradication of human life. Now, we can be pretty confident that Supergirl won't end its third season by killing all the humans. We should probably be more concerned about individual characters, and poor Sam has a target on her back.
Now, Sam has been in danger from the very beginning, when she still didn't know that she was actually an alien destined to become a Worldkiller. She seemed like she had a chance for a happily-ever-after once Team Supergirl appeared to destroy Reign in the previous episode, and things were looking up once she was able to finally be honest and open with her daughter. The reveal in "Make It Reign" that Reign is not only alive but also growing stronger at her expense was a big blow. Sam was circling the drain by the end of the episode as Reign grew even stronger, flying toward the planet's core to begin the terraforming. She volunteered to go back into the Dark Valley to try and cut off Reign's power. Nobody was entirely optimistic that she would survive the experience.
So that's one way Sam could die in the finale. Her body could simply fail after so much stress in such a small amount of time. Obviously preventing Reign and Selena from destroying the world and therefore killing Ruby is something that Sam would be on board with, and her love for her daughter means that it would make sense if she feels the need to sacrifice herself by staying in the Dark Valley and battling Reign to the very end.
Another way Sam could die actually connects to the footage teased in the trailer. Kara is heard angrily declaring that she doesn't kill, presumably not even Reign. If Supergirl intends to let Kara keep her hands clean and not kill anybody but still needs to get rid of Reign for good, letting Sam kill Reign and die in the process could make sense in a sort of "neither can die while the other lives" situation. Maybe killing one means the death of the other.
I know I wouldn't be shocked if we see a confrontation between Sam and Reign before the end of the season, and Sam's odds of survival don't seem all that great. If a major character on the side of the good guys dies in the finale, my money is 100% on Sam with Ruby going to live with Alex and perhaps even Lena reacting to the tragedy by embracing her darker side. That said, my wild card guess for a good guy death is Alura. We can only wait and see what happens.
The Season 3 finale of Supergirl airs on Monday, June 18 at 8 p.m. ET. The show was renewed for a fourth season back in April, so there's probably no need to worry that the Worldkillers really will terraform Earth-38 by the end of the finale, although anything is really possible. There are plenty of Earths in the multiverse, and Supergirl did manage to bring back Argo City in the third season. We shouldn't rule anything out until we get to watch the finale. For what you can watch once Supergirl is finished for the season, check out our summer TV premiere guide and our 2018 Netflix premiere schedule.