Warning: spoilers ahead for the Supergirl midseason finale, called "Reign."
Supergirl spent the first half of Season 3 building the character of Samantha, who fans have known was destined to transform into the supervillain known as Reign. A Worldkiller created by fanatical Kryptonian scientists prior to the planet's destruction, Reign was sent to Earth in a pod much like Kara was, although without her pod being knocked off course for a couple decades. Unlike Kara, however, Samantha was unaware of her heritage until very recently, when she was activated into a Worldkiller by some very nasty people. Samantha is now Reign in all her black-suited glory, and she was unleashed on National City in brutal fashion in the midseason finale. Unfortunately for the civilians, Kara might not be around to help them for a while.
Reign spent much of the episode part-timing as a Worldkiller, only putting on her evildoer duds and frying folks with her heat vision whenever she could escape attention for a bit. She was still able to kill a lot of people during her quick murder trips thanks to her powers, and she didn't face off against an equal -- or who we thought would be her equal -- until the showdown with Kara at the end of the episode. A local fanatic leader proclaimed that Reign was the devil who would bring about the destruction of the world. Although we can't vouch for her status as a devil, I think it's safe to say after the midseason finale that she could cause a lot of destruction unless she's stopped.
In the final showdown between Reign and Kara, we saw a side of Kara rarely shown in this series. She wasn't pulling any punches while battling it out with Reign, and it should have been enough to stop the Worldkiller. Unfortunately, Kara was woefully unprepared for what Reign brought to the table, and not just because Reign sensibly pulled her hair back in a ponytail rather than letting it fly free in her face like Kara. The two punched and kicked and smashed each other with blunt objects all throughout Central City, causing a lot of collateral damage.
Sadly for Kara, the DEO, and the assembled public who saw the smackdown, Reign unleashed unholy hell on the Girl of Steel. It came as a shock when Kara started bleeding from the wounds inflicted on her by Reign. Kara has never taken such serious injuries without kryptonite in play, and even her valiant attempts to get back up and take down Reign failed. She was pummeled in the face, smashed with a car door, and slammed onto enough debris to cause serious injury. Her final stand came to nothing, as they met on a rooftop. Reign finally rendered Kara all but unconscious to the point that she dropped her off the side of a very tall building and Kara was not able to save herself by flying. Neither Martian Manhunter nor Superman was handy to catch her, and Kara hit hard.
When last seen in the midseason finale, Kara was being frantically worked on by medics while Alex begged her to wake up. She showed no sign of returning to consciousness, however, and the trailer for the midseason premiere in 2018 indicates that she's going to be out of commission for a while. Judging by her smackdown of Kara, nobody in National City -- or perhaps Earth-38 -- has the adequate knowledge and skills to take Reign down. Basically, there's a lot of trouble in store for everybody on Kara's Earth, and Reign will probably do her best to live up to her title as "Worldkiller" until Kara wakes up.
The good news is that the Legion of Super-Heroes is on the way, presumably to pitch in en masse while Supergirl is down for the count. We'll have to wait until 2018 to find out, as Supergirl is now entering its winter break. Be sure to check out our midseason TV guide for some important TV dates in the new year.