Spoilers ahead for the December 6 episode of Supernatural, called "Byzantium."
Supernatural fans have seen so many characters killed off over the years that deaths sometimes don't have all that much impact, but it still undoubtedly came as a shock to many when Jack died in the first minutes of "Byzantium." His death was hardly the end of his story for the episode, however, and the rest of the hour was split between Jack's time in Heaven and Team Free Will's attempts to bring him back to life back on Earth.
Both sides ran into complications due to Jack's status as a nephilim, and Castiel wound up making a huge sacrifice that could change everything in the Winchesters' fight against Michael. Here's how it all went down.
Since Jack is half-human, he wasn't sent to The Empty like angels and demons that are killed. He made it to Heaven, where he was able to reunite with Kelly Kline. Kelly was so overjoyed to meet her son that she wasn't even all that bothered to learn that she was dead. Rather, she was sad to learn that her son had died, and it was clear by the look of him that he hadn't been very old.
Back on Earth, Castiel was shocked and alarmed to discover that a distress signal was being broadcast over angel radio, calling for help to come to Heaven. Cas went up to Heaven, where he discovered two angels seriously injured. He learned that Heaven had been invaded by the Shadow from the Empty. The Shadow was looking to drag Jack to The Empty because of his half-angel side, believing that he doesn't belong in Heaven despite having a soul and a human mother.
Luckily, Castiel tracked down Jack and Kelly in Heaven before the Shadow could find them, and Cas explained the situation. Yes, Jack would have to give up a piece of his soul in order to be restored to life on Earth, but a lot of lives would be lost if he remained in Heaven. The Shadow was going to stop at nothing to drag Jack to The Empty, and it certainly had no reason to do Cas any favors.
As fans will remember, Cas got on the Shadow's bad side when he woke up in The Empty, which was supposed to be impossible. The Shadow was not able to go back to sleep so long as Castiel was awake, and Cas was eventually allowed to return to Earth. He'll have to return to The Empty someday given his status as an angel, but as long as he doesn't die and stay dead, he's safely away from the Shadow.
Well, at least he was safely away from the Shadow. As seems to happen to the members of Team Free Will an awful lot, the baddie was able to use his feelings for his loved ones against him, requiring Castiel to either make a huge sacrifice or let the Shadow pull Jack into The Empty for eternity. Anybody who has seen more than a couple episodes of Supernatural can guess which path Cas chose.
Desperate to save Jack from The Empty, Castiel offered himself to the Shadow instead. When the Shadow raised that point that Cas would end up in The Empty someday anyway and therefore there wasn't much incentive to make a deal, Cas reminded it that he could live for eons longer, which means the Shadow staying awake for eons. The Shadow, who presumably hasn't watched the last decade of Supernatural and therefore doesn't know how frequently Castiel dies, decided that a dead Cas sooner rather than later was more appealing than dead Cas eventually.
A deal was struck, but to Castiel's surprise, the Shadow didn't immediately drag him off to The Empty. No, its plan for Cas is far more sinister. The Shadow is going to wait until Cas is at his absolute happiest, and then drag him to The Empty for eternity. The good news is that Castiel was able to return to Earth, where he immediately filled Sam and Dean in on the situation, and they immediately began to work together to find a solution that spares Cas from the Shadow.
Just kidding! Jack's three dads aren't quite that functional, and Cas exacted a promise from Jack that he wouldn't tell the Winchesters about the deal. The real good news is that the angels appreciated Castiel's move that guaranteed the Shadow would leave Heaven, and they had a reward for him: the location of Michael.
Castiel did reveal this tidbit to Sam and Dean, and the three plus Jack began planning a move against the archangel. He hedged when Dean asked how he got the info, but the overall feel was optimism, even if they still had to track down Dark Kaia and her spear to somehow take Michael down before he could kill a ton of people or jump back into Dean or do something equally horrible.
Castiel's sacrifice meant a break in the case of finding Michael, and that is of course a very good thing. Team Free Will wasn't making much headway in efforts to find Michael, so this is a big step forward, even if there was a high price to pay for the info. Unfortunately, Castiel's sacrifice also likely means heartbreak for Sam and Dean in the grand battle against Michael.
Assuming the good guys do beat Michael by the end of the season, Cas would have big reasons to be happy and therefore in a prime emotional state for the Shadow to pull him to The Empty. On top of this, Cas may feel like he has nothing to lose now that he knows his end is coming sooner rather than later. A major shift in the dynamic of Team Free Will may happen in the aftermath of this episode, and nothing good ever comes from one of the boys keeping a big secret from the others.