Warning: spoilers ahead for the "Patience" episode of Supernatural Season 13.
Season 12 of Supernatural came to a heartbreaking conclusion back in May when the finale ended with the death of Castiel. It wasn't the first time that poor Cas died in an episode of Supernatural, but it did seem quite final, as he was stabbed through the back by Lucifer with an angel blade. His wings were even burnt into the ground in the same manner as many angels who died before him on the series. Basically, Castiel seemed really, really, really dead this time around.
Season 13 so far has had the Winchester boys mourning Cas and evidently quite convinced that their angel pal wouldn't be coming back this time. Their hands are also full with Lucifer's nephilim son Jack in the mix. Sam is determined to help Jack while Dean is convinced that Jack is bound to turn evil. For his part, Jack thinks of Castiel as his father rather than Lucifer, as Kelly came to trust and rely on Cas while they were on the run together.
Sam and Dean were at odds enough in "Patience" that they went their separate ways, with Dean joining Jody in setting up her spinoff and Sam staying behind at the bunker to try and train Jack. The brothers got into a big fight at the end when Dean once again lashed out that Castiel might still be alive if not for Jack existing. Jack overheard the fight and Dean's angry point about Castiel, which led him to seemingly use his powers to locate... well, Castiel.
After Jack said his name, Castiel woke up on what seemed to be a dark floor, wearing his iconic trench coat and looking mighty confused. He also didn't seem particularly worse for the wear, although he wasn't on screen long enough to get a good look at him. What feels safe to say is that Jack will be instrumental in bringing Cas back from wherever he is.
A big question that remains is how Cas could actually come back. Angels in Supernatural don't have souls, and they don't go to another plane of existence when they die. Unless they score punishment resurrections that bring them back time and time again, angels tend to just be gone once they die. Perhaps Jack is powerful enough that his attachment to Castiel kept Cas in a void of sorts between life and nothingness. Cas definitely seemed to be alone, which would make sense if he's still around because of a newborn nephilim.
We also can't say just how Jack might be able to bring Cas back from where he is now. It's unlikely that Supernatural will keep Cas separated from the rest of the main cast for too long. We do know that Castiel will have a part to play in the Scooby-Doo crossover, and that crossover may happen due to Jack using his powers. Who knows? Maybe the Winchester boys will cross paths with Castiel in Scooby-Doo land and yank him back to the real world after they escape animation. Stranger things have happened on Supernatural.
We'll have to wait and see. New episodes of Supernatural air on Thursdays at 8 p.m. ET on The CW.