Spoilers ahead for the Season 13 premiere of Supernatural. You may want to come back later if you haven't watched the episode just yet.
Supernatural is the CW series that just keeps on going, and it has officially returned for Season 13. The episode was surprisingly somber and mournful for a Supernatural premiere, which are usually more high-octane. Given what happened in the Season 12 finale, however, the more somber tone was just right. The WInchester boys are mourning the losses of Castiel and their mother while they try and track down Jack, a.k.a. the nephilim son of Lucifer and Kelly. Although Mary was still alive when last they saw her, Dean has given her up for dead due to the fact that she's trapped with Lucifer, and Lucifer has been arguably their biggest bad to date.
Given the events of the premiere, it may be safe to say that Lucifer is not the biggest threat to the Winchesters (and humanity) anymore. That threat may in fact be young Jack. Supernatural actually spent a healthy chunk of the second half of Season 12 explaining why nephilim are powerful abominations that should not be allowed to exist, so fans were probably more than ready to dismiss Jack as a villain. The Season 13 premiere revealed that Jack is really a child, despite being played by the 27-year-old Alexander Calvert, and he considers Castiel to be his father because of Kelly's bond with Cas.
Where the issue gets sticky is that Jack seems to lash out with his powers whenever a certain something rubs him the wrong way, and his powers are substantial. He's too young and naive to know when to control himself, and Dean at least isn't really in a frame of mind to show tact. If Jack decides that the Winchester brothers are his enemies, he could easily destroy them and what little they have left in the world. Considering that they decided to bring him back to the bunker at the end of the episode, he'll have easy access to just about everything. Supernatural star Jared Padelecki had this to say about why Jack is such a big deal:
Jared Padalecki's comments to Variety indicate that Jack the nephilim is evidently destined to be more powerful than even Lucifer, and that does not bode well for our boys, as they currently do not know how to kill a nephilim. Being stabbed in the chest by an angel blade should have incapacitated him, but he plucked it out of his heart like it was nothing. It's quite possible that holy oil won't be able to contain him. Dean intends to use all their resources to try and figure out how to put Jack down.
For his part, Sam clearly hopes that Jack can be redeemed in Dean's eyes so they won't have to kill him. Unfortunately, the odds aren't really in his favor. Dean is a man with a plan motivated by what he believes to be the deaths of his mom and his best angel pal, and the losses of Mary and Castiel mean that Sam doesn't really have any other allies who might be able to talk Dean down. There's also the point that Jack might have been controllable with Kelly, Castiel, or Lucifer on hand to set him on a certain path. With Kelly and Cas dead and Lucifer trapped in another reality with Mary, it's really just Sam, Dean, and Jack.
You can catch new episodes of Supernatural on Thursdays at 8 p.m. ET on The CW. If you're still looking for what to watch on the other nights of the week, take a look at our fall TV guide.