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Cas strongly believes in the many-worlds theory.
He can almost feel them sometimes, the alternative worlds that co-exist amongst his. He thinks that if he closes his eyes and meditates for long enough, then maybe he would be able to actually see them.
He’s certain there’s multitude of worlds where the apocalypse runs rampant, whether the original one that was planned from the beginning, where in one verse Lucifer rules and in the other, Michael, where both Sam and Dean had said yes and destroyed the world. Or perhaps there’s a world where Dean said no, but couldn’t snap Sam out of Lucifer’s grasp, a world where Cas and Dean are left to deal with the consequences of the apocalypse – because Cas has no doubt that any version of him would stay with Dean to the ends of the Earth, even if it meant losing everything.
In another universe, it’s Raphael’s apocalypse that flung the Earth into ruination – a world where Team Free Will had failed to prevent it. He’s unsure if they’d have survived for long if Raphael had gained that much power from purgatory, but he thinks Raphael would have kept him alive for his insubordinate behaviour, just to torture him with the prolonged suffering of the Winchesters.
Cas thinks there’s probably a world where he had retained his God status and had destroyed the world he had given his life to protect, or a world where Leviathans gradually consume the entirety of the planet before moving on to other parts of the solar system, and then maybe even further if they worked out a way for them to devour the universe.
He believes there’s a universe where in Lucifer’s crypts, the scene had unfolded differently. That he had realized the importance of words, the weight of the sands of time on a human. Instead of telling Dean that he didn't know what pulled him from the haze of hypnosis, Cas would have told him that it had been Dean that broke him out of it. He can’t be sure, but he thinks that maybe, there’s a universe where Cas outwardly acknowledged how much it meant for Dean to admit that he needed him, and Dean would give him an awkward smile. Cas would have told him that he could read in-between the lines and read the unspoken I love you.
He knows there may be a universe where he’s still human, a world where perhaps Dean hadn't cast him from the bunker, and in a fit of human hormones mixed with the full force of emotions that he was usually sheltered from by his grace, he would have admitted his feelings for Dean.
From that universe, stems another where Dean rejects him because his father’s influence on his sense of manhood still lingers, because in that universe John Winchester is all forms of downright cruel and hateful instead of simply negligent and thoughtlessly angry. But there’s another universe where Dean accepts him, and admits that he may feel the same way. He thinks, that in that universe, they could be happy, at least for a while.
It’s in a conjoined universe where Dean refuses to give up hunting for Cas - he will still love Cas, they would still be a couple but Dean refuses to let people get hurt when they could prevent it. But with Cas left human and vulnerable during hunts, Dean is distracted and not as focussed on the monsters or demons as he should be and it would be his downfall. He’d die in Sam’s arms as Cas stood by and wondered how to fix it, no longer angel enough to heal him, but not human enough to be unable to feel Sam’s silent accusation as he held his dying brother to his chest and sobbed quietly.
The alternate universe to that one is where Dean gives up hunting, decides that it’s time for the younger hunters to take over and for the brothers to become more like Bobby, researchers to help the other hunters with any issues they had. Sam would find someone and settle down and get married, they’d have children and Dean would remember how much he loved looking after kids. He’d even suggest that he and Cas could maybe adopt. They’d have a little garden and Dean would reluctantly allow him to keep some bees.
Grimly, he realizes that there’s very possibly a universe where he was not the one to raise Dean from perdition. He would be just a nameless angel dragged down to Hell to save the Righteous Man and his life would be spent meaninglessly serving a heaven that didn’t care and an absent father that no one remembered.
He wonders if in that world, they’d still be brought tumbling into each other, over and over again - so many times they’d been torn apart, only to be shoved back together. He wonders if he would still rebel against heaven for the Winchesters, but ultimately fall from grace for Dean.
It’s then his thoughts start to drift to less plausible, but still somehow charming ideas. He’d like to think there's a number of universes where he’d been born human somehow, that the Winchesters hadn't been given such a tragic destiny to fulfil that would shred their family to pieces, killing their mother and leading their father to become abusive in neglect of his sons because of a pointless quest for revenge.
It with a sharp kick to his side that he's brought out of his self-induced trance. He feels his ribs crack and he knows that those universes are a long way from his. There’s an emptiness - a lack of feeling, in Dean's eyes that chills Cas to his core.
In his universe, Dean pummels him to the point where his weakened grace can no longer heal him without him consciously willing it to, but it would take too much energy to do so much healing in so little time, energy that he doesn't have at expense. Dean raises the angel blade, his hand is shaking but Cas accepts that this is going to be his end.
- -
Dean slams it down next to Cas’ head and he leaves, making a promise not to miss the next time.
In another universe, Dean had not missed. He would either mourn (he would repent or he would self-destruct) or he wouldn't and he'd become a demon and bring the Earth to complete desolation with nothing but his hands and the First Blade.
Cas knows that there has to be a universe where he will be able to speak through the pain when Dean crushes his bones, a universe where he can't take the heavy silence between him and Dean any longer - nothing but longing and things left unsaid floating between them. Blood won’t be stuck in his throat and clogging his lungs and sticking to his lips, he will be able to utter what his Dean couldn't in his universe when the tides were reversed.
I love you.
He'd like to think that the Dean of that universe would freeze up and drop the blade, utterly horrified at his actions. He'd probably run away, afraid that he’d hurt either Sam or Cas again. Or perhaps, his admission would have no effect - perhaps it’d solidify his resolve and he’d plunge the blade straight into Castiel’s heart and watch, detached, as his remaining grace burnt through the ground and dissipated throughout the Earth’s crust.
It doesn't matter anyway, he realizes as he lay near the pile of Dean’s unwanted possessions, battered and broken.
What happens in other universes has no effect on his and he’s too tired, too shattered to give it any more thought.