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It wasn't supposed to be like this because angels can't turn. Cas told him when all the other angels started to zap back to heaven that it wasn't for self-preservation. There was just no reason to interfere anymore on a doomed little planet that was about to burn itself out.
"Oh, good," Dean had said. "That makes me feel better."
Cas had leaned in and pressed their foreheads together, his hand curling around the back of Dean's neck, squeezing gently, a reassuring presence. "They've been wrong before," Cas told him. "They have a habit of underestimating you." Dean wasn't sure if he meant humankind in general or Dean specifically, but Cas had kissed away the question before he could ask it.
Dean's used to this by now. After three months, he can see it coming. Cas's eyes will lose focus, and he'll trail off in the middle of a sentence. Dean's got about ten seconds to act before the muscle spasms, the stiffening limbs, the cloudy eyes, and those terrifying moments where Castiel isn't Castiel anymore, but a clawing, grasping thing with just one goal: ripping any breathing thing it can get its hands on to shreds.
"Hey," Dean says, grabbing Cas by the chin, trying to get his attention. But Cas twists, spine arching, and that's it. He's gone. Dean grabs his wrists and pins them to the bed, settling his weight on Cas's hips.
"Come on, Cas," he says as Cas writhes beneath him. Used to be this was Dean's idea of a perfect afternoon. Fucking zombies fuck everything up, every time.
Dean gives a shout for Sam because the last couple of weeks, it's taken both of them to restrain him more often than not. Dean doesn't know what the fuck they're going to do when Cas is strong enough to throw them both off like this.
They thought it would be better if they stayed in one place, so they went back to Bobby's, even though Dean had never wanted to set foot in that house again after what happened. And it had helped for a bit. Cas could rest when he needed to instead of having to press on for days while they looked for other survivors, but it was really just delaying the inevitable. With no outside contact and hordes coming out of the city every few days, there was no two ways about it. This was the end of the line.
Dean can see Cas fighting it, which is a good sign. Sometimes it takes a while to get through to him. Now they just have to wait it out. When Sam appears in the doorway, gun in hand, Dean shakes his head.
"We're okay."
Sam hesitates. Sam always hesitates lately, probably because he's willing to admit what Dean isn't: that one of these times, Cas is going to be too weak, too exhausted to fight it off, and then they're going to have to deal with it.
"Put it away, Sammy. We don't need it. Right, Cas? We're fine."
Dean can feel the muscles in Cas's arms tensing beneath his hands, his fingers twisting, reaching up, seeking out something to tangle in and tear, and Dean presses down harder, does his best not to flinch when Cas cranes his neck, mouth open wide, an unearthly growl spilling from the back of his throat.
"We're not doing this now. You hear me, Cas? So you gotta fight. I know you're tired, but you gotta fight, okay?"
Dean had been ready to give up before Cas was attacked. You can only live through so many apocalypses before they take the fight out of you. He knew Cas could see it and was grateful when he said nothing. Cas had promised enough times in enough fucked up universes that he was ready to check out when Dean was.
But then Cas had appeared in the usual rush of wings, and before Dean could ask where the hell he'd been for the last three days, Cas collapsed, his coat torn and streaked with bloody, grimy handprints, and just where Cas's neck met his left shoulder, a bite, still fresh and oozing.
Everything had changed, then. Angels can't turn, but vessels can. And for a fully powered up angel, that isn't a problem. But Cas ain't exactly running on a full tank these days, and they don't exactly have their pick of new, shiny clean vessels in the middle of the zombocalypse. Now all it takes is a little lack of focus, a little overexertion, and Cas starts to slip away. So this is just how it is now, this cycle. Cas is weak, so he starts to slip. Then he fights it off, leaving him weaker than before, making it all the easier for this thing to creep up on him. Strange that only when the Winchesters are well and truly fucked, left with zero options, that they refuse to give in.
It takes a few minutes, but Castiel's head drops back against the bed and his eyes squeeze shut. Dean can feel him start to shake, sucking in quick, panicked breaths.
"That's it. Just breathe. You got this."
Dean counts the seconds. It's longer and longer each time. He keeps track. He knows Sam does, too, but they never talk about it. There's nothing to discuss. They can't fix this, and there isn't anyone else to try.
Cas relaxes ever so slightly beneath him, no longer struggling against Dean's grip, but still tense and taut. When he opens his eyes, they're clear and focused again, and Dean lets out a long breath.
"Cas?" Dean says carefully, but Cas flinches at the sound.
"M'okay," he mumbles.
"You sure?"
Cas flexes weakly against Dean's hold. "Let me up."
The second Dean eases his weight off of him, he scrambles back across the sheets and off the other side of the bed, leaning against the wall for support, shaking with the effort of keeping himself upright.
Dean glances over his shoulder at Sam only to find him with his gun lowered but still pointing in their direction. "Put it away. He's fine."
"I'm okay," Cas says again, his voice raw and barely audible even in the silence of the room.
Sam waits a moment longer before giving Dean that look that says shout if you need anything and heading back downstairs.
The second Sam is gone, Cas slumps to the floor, back against the wall, knees drawn up, his hands covering his head. It's a long time before Dean chances sitting beside him and longer still before he reaches out a hand and rests it on Cas's back. Each breath Cas takes shudders in his ribs.
Dean rubs slow circles between Cas's shoulder blades and waits it out with him.
"It's only going to get worse," Cas says, voice muffled by his arms. "The angels are gone, and I'm only going to get weaker. It won't be long before I can't fight it anymore."
"You don't know that," Dean says. He pushes his fingers into Cas's hair, brushes his thumb across the spot behind Cas's ear and feels him shiver and uncoil just a bit.
"But you do. You saw it before."
"That was different. That was some bullshit that Zach--"
"It wasn't. Don't lie because you think it will make me feel better. It doesn't help."
"Then what does help?"
Cas only shakes his head beneath Dean's hand, and he must still be out of it if he's not aware of every bit of them that's currently touching. Castiel doesn't like this anymore, the touching, too worried about any chance that he could infect Dean, no matter how miniscule, too worried that he might start to turn when Dean's guard is down. It had been fucking bizarre to go from whatever they were to whatever they are, and Cas learning all the ways something as simple as skin against skin can do so much. Now, Cas barely lets Dean sit beside him for too long, and if getting used to being close to Cas had been strange, relearning life with a safe foot and a half between them at all times is even worse. Dean takes advantage of the momentary lapse in vigilance.
"I don't like being like this. I've seen what they do, and I don't... I want you to--"
"I'm not putting a bullet in your head, so don't ask."
"You did it for Bobby."
They'd kept him in the panic room, chained and locked up like a fucking monster (and he was, but jesus, he still looked so much like Bobby), back when there was still a chance, back when there was something as foolish as hope. Back when they thought there was still a way out of this because somehow they always found a way out of shit like this. But it was selfish and stupid, and eventually Dean loaded his shotgun, said his goodbyes to deaf ears, and ended it.
Dean drags a hand down his face and takes a deep breath. "Yeah. I did. So you understand why I can't do that again."
Cas turns his head on his arms and watches him for a moment. "Sam would--"
"No, he won't," Dean says. He could, but he won't. Dean won't let him.
"Then I'll leave," Cas says.
"No. Stop it, will you?" Dean tugs at Cas's wrist until he relaxes enough to let Dean curl their fingers together, as if that will keep him from zapping off to who knows where. "Listen. You zap off somewhere, then I'm gonna end up dragging Sam over half the country trying to find you and getting us both killed. So just stay. Save me the aggravation. And the brutal death."
"Stop." Cas tries to pull away, but Dean won't let him.
Cas looks up at him for a long time, his thumb brushing up and down Dean's absently. "I have spent the last few years doing everything in my power to keep you and Sam safe. I'm not going to stop now, just because I'm the threat."
"Look around you, man. Nothing is safe anymore. No one's coming back from this. Don't you get that? This is it. So stay. Because things are shit, and you running off isn't going to make that any better."
"If I hurt you, I'd never forgive myself."
"To be fair, if it got to the point where you were out of it enough to hurt me, you probably wouldn't be too concerned about forgiving yourself after. And I'd probably be a zombie or dead so my guess is it wouldn't bother me too much either."
"Dean, I'm serious."
"Come on, it's funny," Dean says, squeezing his fingers. "It's the end of the world. Laugh."
Cas shakes his head and sighs. "It's always the end of the world, and it's never funny," he says.
And it's true. There's always been something, and he knows it's the life, that this is what comes with hunting, doomsday always nipping at your heels. Doesn't make it fair.
"I know," Dean says, turning to press a kiss into Cas's hair. "Believe me, I know."