A/N: What what what am I doing? I have two multi-chapter fics waiting for updates. Why am I writing a Destiel one-shot? Oh, you know how it is. When a plot bunny gets its teeth in you there's nothing you can do.

By the way, I didn't address this in the fic, but I can't imagine why they didn't just carve the sigil into one of the boys. It would have had the same effect without the risk of blowing Cas all to hell. Oh, well. Plot holes are acceptable if they mean we get to see Misha Collins's chest.

The box cutter is heavy in his hand, heavier than it has any right to be, as Castiel unbuttons his shirt. It's not as if he is a stranger to pain, especially the muted pain that filters through his vessel. Still, the prospect of what he is about to do is not exactly an attractive one.

"Uh, Cas?" Dean says hesitantly as he watches the angel begin to disrobe, "What are you doing?" The confusion on his face is almost comical. It's clear that he has not yet caught on to Cas's plan, that he's imagining that Cas is about to strip and rush into the warehouse naked and wielding only the box cutter. He swallows hard as Cas pulls open his shirt, and suddenly he has trouble meeting Cas's eyes.

Castiel waits for Dean to stop staring before he replies, "I am affording you an opportunity at great personal cost. Please don't waste it." Almost casually, he presses the blade of the box cutter to his chest and draws the first arc of the circle that will become the banishing sigil. It's easy at first. The blade is sharper than he expected, and he cuts deeper than he intended. It's almost gratifying to see blood well up in the cut and immediately begin to trickle down his chest, and it's certainly amusing to see the boys' horrified reactions. But he doesn't have to work very hard to suppress a smile at the squeamish noise Sam is making, or Dean's, "Jesus fuck, dude!" because that's when he notices the pain.

It was almost as if his body can't believe what he has just done to it, as if it's been holding back the sensation of parting flesh until it has verified the wound. But now it creeps in insidiously, making the cut feel twice as deep, twice as wide, and filled with acid. A harsh, biologically-ingrained reminder that box cutters are not for skin. He should have expected it, he realizes. As his powers have waned, he has become more and more human-like, more and more tied to his vessel. Of course his perception of physical pain has changed as well. What had once been a quiet reminder that his vessel needed repairs has become a serious nuisance.

The only outward sign he shows is a slightly longer exhale than normal, and a series of startled blinks. But Dean notices. And he has figured out what Castiel is trying to do. "Seriously?" he says, "I know we've been getting creative with where we put those sigils, but this is getting a little crazy."

Sam chimes in, "Dean's right. Do you even know what'll happen if you activate one of those things when it's carved into your skin?"

"I can't say I've ever heard of anyone trying it," Castiel replies, "But if you have a better idea, I would very much like to hear it." He keeps his voice dismissive, but he's glad for the interruption. To his chagrin, he finds himself hesitant to make the next cut. He hadn't expected this to be a problem, but pain has turned out to be a swift teacher and already his mind screams at him to drop the knife and never pick it up again.

The box cutter is so heavy now that it's practically a physical effort to raise it. Castiel feels as though he has to tense every muscle in his arm to hold it steady, and even then it's a battle of will to press the tip to the corner of the bleeding gash just inside his left nipple. His own weakness is humiliating. He has seen the boys sew themselves up after fights without so much as flinching; surely he can finish this sigil.

This time, there is no delay between the cut and the flare of agony. Each second is a struggle not to pull away, not to ease off the pressure. He fights with himself for every inch. When he finally gives up, panting now, the second cut is half as long as the first and not as deep. It bleeds just as much though.

"Cas…" Dean says, almost reaching out to him, but stopping before his arm really leaves his side. His brow is creased, and his breathing matches Cas's breath for breath.

Castiel doesn't give him time to say anything. He tries to recreate the confidence he had felt before he had started, the certainty that had allowed him to draw that first line without hesitation. With a deep breath, he divorces himself from fear of pain and slashes the next line quickly. Then, before his instincts can instruct his arm otherwise, he makes two more swift cuts and closes the circle.

Each cut doesn't just add to the pain; it multiplies it. Castiel doesn't even have time to feel satisfied before his face grows hot, then cold, and then he staggers back and leans against the wall of the warehouse. His next breath comes out ragged, almost a groan, but not quite.

He is so busy blinking back the spots in front of his eyes that it takes him a moment to realize that Dean is there, holding him up with one hand on his shoulder. The weight of the box cutter is gone from his hand. For a moment he thinks he's dropped it, but then he sees Dean flicking the blade closed and putting it in his pocket. "Stop it, Cas," says Dean, "You don't need to do this. Just heal yourself up and we'll fight our way through together."

Castiel glares up at Dean and snatches the box cutter back, his presence of mind restored. "And who will save Adam when we're all killed?" he challenges, "This will give the mission the best chance for success."

Dean blows an exasperated breath through his teeth. "Cas, even if this goes exactly as planned – which, I'm sorry to point out, almost never happens with our plans – you don't even know if you'll survive."

"Dean," Castiel replies slowly, "You are about to give your consent to Michael. Very shortly, you will be so busy dealing with the fact that an Archangel has taken over your own personal steering wheel that you will have very little mental capacity left to worry about any of the things or people that you currently care for." He leans forward a little to hiss into Dean's ear, "As someone who, soon, will not care whether I am alive or dead, you have no say in how I choose to meet my end."

"So this is a suicide run!" Dean spits, furious.

Castiel looks straight into his eyes and says, "If you say 'yes' to Michael, then I sincerely hope so." He tries to tell himself that it's because he doesn't want to live in the world after Michael has turned it into his own personal playground, even as the truth whispers itself in his ear. Castiel has lived with Michael before, and could do it again. What he can't do is live in a world without Dean Winchester.

He raises the knife once more, but this time he can't make himself do it. His hand begins to shake, making the tip of the blade bounce and scratch against his chest. His other hand rises to steady it, but if anything the shaking only gets worse. Dean is watching him with something like pity behind the guilt that is always, always there in his eyes, and it's enough to make Castiel steel his nerve and press the blade to skin once more.

Except he flinches away from it at the last second, involuntarily, as if the flesh being cut and the flesh doing the cutting were two different beings. The blade's tip stutters over his skin, leaving only a superficial mark that doesn't even bleed. He can't help it. He has always thought himself brave, but apparently ideological courage can't help someone overcome a primal aversion to pain. Castiel drops his hands to his sides, defeated, before offering the blade to Dean. "You'll have to finish it," he says.

Dean scoffs loudly, almost a bitter laugh. "Like hell I will," he says. Even though Cas has regained his footing for some time now, Dean's hand is still on his shoulder.

Castiel shifts his gaze over Dean's shoulder and holds out the box cutter. "Sam, then," he says to the man who has been standing back and looking extremely uncomfortable.

Sam's eyes flick to the box cutter, then to his brother, then back to Cas. "I don't think…" he stammers, "Maybe if…" Then he's silent for several seconds before finally striding forward and taking the box cutter from Castiel's hand with a resigned, "Okay."

"SAM!" Dean bellows, making Sam shrug helplessly.

"Dude, what else are we gonna do?" Sam says, "Charge in there like Butch and Sundance? Stand out here for the next hour while Cas works up the nerve to stab himself a few more times? Leave Adam to become Michael's bitch? From where I'm standing, this…" He holds up the box cutter. "…seems like the least-awful option."

Dean turns to stare into Cas's eyes, and Cas does his best to look determined. Even so, he is fairly sure that Dean can see in his eyes the fear, longing, pain, and the other pain, the deeper one, the one that had started when Dean first declared his intention to give himself to Michael, and that has only gotten worse the closer they come to the fateful moment. In a glance, they silently exchange all their arguments and all their begging, Dean wishing Cas would save himself and Cas wishing Dean would do the same. Castiel notes that they had become skilled at saying volumes with just a lingering glance, which is good for two men who are generally bad at talking about their feelings aloud. They understand each other with that glance.

Too bad it doesn't change anything.

"I'll do it," says Dean, finally breaking his eyes away and looking at Sam. Sam hands the box cutter over almost gratefully. Dean turns the thing over in his hands a few times before he rolls out the blade still dripping with blood. As they stand there for a long, loaded moment, a dark droplet rolls off the tip and scatters on Dean's shoe. Castiel wonders if the bloodstain will still be there when it has become Michael's shoe.

Then Dean, looking as miserable as if it were his own blood being spilled, shifts his grip on Castiel's shoulder so his hand is on Cas's chest, just above and to the side of the gory circle. The pressure pins Cas to the wall. "Hold still," Dean mutters as he begins to work.

Dean's slashes are quick and unhesitating, and the symbol in the center of the circle is finished before Castiel has time to grab Dean's wrist, holding his hand tighter against his chest. It's now the only thing holding him up, because his knees are buckling under him at this fresh blossoming of anguish. Dean ignores the way Cas's head is thrown back against the wall, his teeth gritted and his eyes screwed shut, as he twists the knife to make the little loops and zig-zags around the base of the sigil. It looks brutal, but even through the haze Cas can recognize it as the kindness that it is. He knows that Dean knows that the faster it's over, the easier it will be.

And it is over quickly. Moments later, as the initial roar of pain subsides to a persistent sting, Castiel opens his eyes to find his right hand still clamped around Dean's wrist, and his left tugging on a handful of Dean's shirt. He doesn't even remember grabbing it.

"You okay?" Dean asks, his voice actually shaking. Castiel belatedly remembers what Dean was forced to do in Hell, and realizes that he should never have asked this of him.

Their eyes meet again, and there is guilt in both of them now. Castiel tries to stay angry at Dean, tries to remind himself that Dean is about to betray them all, but he can't. Not when Dean's eyes are telling him what neither of them have the courage to say. This is the last time we are going to see each other.

Castiel doesn't want to leave it like this, doesn't want to make pain the last thing they ever inflict on each other. But what else can he do? What else is there to say? There is no time to say all the things he wants to say, and not nearly enough time to do all the things he wants to do.

But there is time for a few words. There is time for a kiss goodbye. The thought springs up unbidden, but it lingers, electrifying their gaze and Dean's hand on Cas's bare skin. There is time, if Castiel has courage enough to use it.

But as with the blade, he finds himself flinching away from the pain he knows is waiting for him. He can't make himself do it.

So once again, Dean does it for him, tipping Cas's chin up for a little, stolen kiss. The world stops on that kiss, soft and shy, more like a lovestruck teenager than a grown man full of hubris, and suddenly Cas sees that he has all the time in the world. Enough time to tell Dean, in the movement of their lips, how long Cas has loved him. What, exactly, it meant to Cas to give up everything he had known in his thousands of lifetimes for a chance to stay near Dean for the length of his fragile human span. How much it hurts to fall, and that even so, he wouldn't give it up for all his powers back. And that the only reason he isn't now wrapping his arms around Dean and pressing their bodies together and never letting go is because it would smear the sigil.

By the time Dean pulls away, Castiel finds that he has nothing left that he needs to say. They disengage slowly, uncurling their fingers from their grips on fabric and flesh. Castiel turns, hides the sigil behind his shirt, and opens the heavy warehouse door.

As the door closes behind him, he hears Sam start to say, "Uhhh…"

Dean voice is back to its usual cocky timbre as he quickly cuts Sam off with a defensive, "What? A dying man deserves a last kiss."

Sam ignores the obvious objection for the more diplomatic, "Dean, he might not die."

"I wasn't talking about Cas." Castiel can barely hear Dean's voice through the door, and then it's dark and silent in the warehouse. He is alone.

But not for long.

Castiel has felt the effects of a banishing sigil before, like being hit by a brick wall. Except this time the wall is inside of him, expanding outward, shredding his grace on its way to its intended targets. He expects it to hurt, but the pain is somehow gone in the center of that blinding light. The only sensation that remains is Dean's kiss lingering on his lips.

Then even that is gone, and Castiel resigns himself to the fact that in spite of everything, they have failed. Not that it matters to him anymore, since he is fairly sure that he is in the process of dying.

He doesn't hope to survive. He doesn't even hope for the Earth he is leaving behind. His one hope is for Dean: that maybe, just maybe, after Michael has had his fill of holy war and righteous destruction, he will let Dean go free in what remains of the world.

It's a small, remote, stupid hope, but Castiel clings to it as he spins into the void.