WARNING: The following article contains spoilers for Season 15, Episode 9 of Supernatural, "The Trap."
Supernatural's midseason premiere throws the Winchester boys through a loop. Not only is Dean forced to return to the grim woods of Purgatory where he nearly loses Castiel to a gang of hungry Leviathans, but God/Chuck gives Sam a harrowing preview of what lies ahead should the brothers defeat him.
In this dark future, the absence of Chuck's "light" leads to a monster uprising far beyond the hunter community's control -- something Dean describes as being less like hunting and more like "whack-a-mole." As their numbers fall, Sam and Dean eventually fall prey to vampire bites and are slain, like any other monster, by Jody Mills and Bobby Singer in a fight to the death. In the end, present-day Sam is deterred by what he sees from going ahead with his, Dean and Castiel's plan to cage Chuck using a powerful spell. In doing this, he rationalizes that he's trading victory for the lives of himself and those he loves. In future Castiel's case, this fate would have been one of the worst imaginable.
We're still a little vague on the details about the spell to bind Chuck -- given to Dean and Castiel by the archangel Michael -- but it seems to require someone to bear a Mark in order for it to work. When Dean and Castiel are preparing it, we learn than Dean is incapable of being the bearer as he previously bore the Mark of Cain. This implies that this spell's own Mark works in a similarly corruptive fashion, which is confirmed by what we know happens to Castiel, who volunteers himself up to take it.
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In the timeline that Sam gets a glimpse into, we learn that the Mark caused Castiel to lose his sanity and, as a result, the Winchesters sealed him in a Ma'lak box. This box was crafted by Dean when he was being possessed by Apocalypse World's Michael in Season 14. Adorned with sigil engravings, it's strong enough to keep any creature locked in it trapped forever. Dean planned on sealing himself inside it in a self-imposed exile to contain the world-ending being inside of him. It was later used on the Nephilim Jack, instead, after his lack of a soul causes him to become too destructive. Effectively, the box is a coffin to be buried alive in, and very much a last resort.
When Sam gives up on the plan to the use the binding spell in the present, Chuck crushes the orb containing the spell's magic -- freeing Castiel of the Mark. By skirting this future, Sam has rescued Castiel from one of the show's worst forms of punishment. But in order to do this, he also had to forfeit their best bet to stop Chuck, meaning that it's more than likely some other, nasty fate is being lined up instead by the angel's vindictive creator.
Airing Thursdays at 8 p.m. ET/PT on The CW, the final season of Supernatural stars Jensen Ackles, Jared Padalecki, Misha Collins and Alexander Calvert.