Of course – of course the one thing that would snap Dean out of the Blade's thrall is the death of his little brother.
Even though – perhaps especially because that death is at his own hand.
Sam's eyes stare up, wide and shocked, unnaturally bright; bright red blood bubbles past his lips as Dean sinks to his knees beside him, the Blade falling from his shaking hand.
"Sam?" Dean whispers, shaky and lost. "Sammy?"
Castiel pulls himself up off the floor a few yards away. The Blade and the Mark have somehow imbued Dean with an unnatural level of strength, and a single blow sent the weakened angel flying. He stumbles back to where Dean is now holding his brother in his arms, tears streaking his face as he sobs out "Sam" and "no" and "sorry" again and again, panic in his voice as it rises in speed and pitch.
He looks up at Cas with desperate, pleading eyes. "You gotta help him, Cas," he whispers, his voice trembling, breaking. "I didn't mean to – please, you gotta help him!"
Castiel kneels on the floor beside Sam, wincing and holding an arm across his aching ribs. His heart sinks as he sees that the light has already gone out of Sam's eyes, his body still, lungs no longer struggling for breath.
He's gone.
Castiel's grace is waning, nearly completely depleted at this point, but he thinks perhaps he has just enough left to bring Sam back. He reaches out a hand and places it gently against Sam's head, closing his eyes and focusing every shred of power he has left on healing the damage the First Blade has done.
A searing heat flows from deep in his chest, through his hand and into Sam, and vivid images fill Castiel's mind – images of past moments he has shared with the younger Winchester.
"I know you're still one of us…"
"Everything you did, you did it to help…"
"This is the part where you hug back, Cas…"
Tears spring to Castiel's eyes, and he finds himself sending out a desperate prayer, despite the fact that he doesn't really think anyone is listening anymore – a prayer that he has enough strength left to help the one person who's always believed in him, from the moment they first met – even when he ceased to believe in anything, himself. The person who's been a friend to him when no one else would be.
The person that he loves, he realizes, abruptly and completely – and perhaps too late.
Please… please, even if it's all I've got… even if it means my life… please, don't let him be gone…
Sam draws in a deep, desperate breath – and Castiel collapses to the floor, gasping himself, a bright, burning pain consuming his entire being. Everything hurts, and Castiel is certain that he's dying – again – until slowly, far too slowly, the pain fades out, leaving him aware only of his harsh, panting breaths… the thudding beat of his heart in his chest…
… the sudden, unmistakable awareness of everything that means that he is…
… human.
Dean clutches at Sam, sobbing like a broken child, making promises and begging forgiveness. But Sam is staring at Cas, eyes wide and wondering, as if seeing him for the first time – and suddenly Castiel wonders just how much he shared with the younger Winchester, with the last of his grace. Finally, Sam's eyes lower to the Blade on the floor, and Cas remembers that this isn't over yet.
He reaches down and picks it up, watching Dean warily.
Dean doesn't even notice.
It's as Castiel suspected; Dean has fulfilled the Mark's requirement of him – he killed his younger brother, as Cain did millennia ago. Its hold over him should be gone – and Castiel thinks if he could see Dean's arm right now, he'd see that it's clear and whole, no longer seared with the Mark of Cain.
The Mark gone – the blade nothing more than an old scrap of bone.
And Castiel… alive, miraculously… but an angel no longer.
He busies himself putting the Blade away in a safe place, nonetheless, locked into a box that's warded against being opened by any demon or angel – not because he thinks it's still dangerous, but because he wants to give Sam and Dean a chance to talk. And they do, about all the things they should have dealt with so long ago, clearing the air between them and mending the parts of their relationship that are broken and damaged.
It's hours later when he's in the kitchen, doing his best to figure out how to make them something to eat, that he feels Sam enter before he hears or sees him – turns on his heel to see him standing in the doorway.
Suddenly, he can't speak. His mouth is dry, his heart racing, his mind acutely aware of Sam's eyes, intent and inscrutable, focused completely on Castiel. He swallows hard, lips parting, trying to think of something to say – but Sam is already headed toward him, his stride sure and swift. Before Cas can react, Sam puts an arm around his waist, pulling him in close and off balance. Sam's other hand cups the back of Cas's head – strong and firm, holding him in place as Sam lowers his head and captures Cas's mouth with his own.
Cas freezes, stunned and disbelieving – and then melts under the heat of Sam's gentle onslaught, his own shaking hands rising to Sam's shoulders as he finally regains enough presence of mind to return the kiss.
This, he knows he's good at. He doesn't know much about… much, but he knows he knows how to kiss.
If the warm, soft, pleased smile on Sam's lips is any indication, when he finally pulls away, leaving Cas breathless and wanting.
Sam's voice is hushed and low, his eyes dark with want and warm with affection, as he leans in and speaks into Cas's ear.
"I love you, too."