A/N: This was written for vail_kagami's awesome prompt at the ohsam LJ comm during the Sam-week comment fic meme.

Warnings: SPOILERS for season 6 up till 6.18: Frontierland. Gore, violence, much creepiness, mild swearing. Also, twisted!Cas.

Disclaimer: I don't own Supernatural or any of its characters.

Unravelling

Every time Castiel comes back, there's something missing.

It's always a room: plain and brown, unfurnished save for a wooden chair in the centre. Sam sits in that chair – relaxed, open, trusting. There is none of that defiance that Castiel is used to seeing; no upturned chin, no guarded look. Sam merely is as he sits, and he thrums with power.

Castiel reaches out, feels a thread of that power connecting him to Sam, an endless loop of energy that shimmers like gossamer between them. Sam's hand unravels, flesh and muscle and bone wisping at the edges before joining the thread.

Sam unspools, Castiel absorbs.

When he feels like he's taken enough—

(there'll never be enough)

—he takes a deep breath, breaks the connection and re-emerges out of Sam's mind.

Sam's lying panting on his bed, betrayal in his eyes and blood on his lips.

"I'm sorry," Castiel says, passes a hand over Sam's forehead and makes him forget.

"So, I'm thinking," Dean says, "if this Mother chick is really creating these new monsters, we're better off sticking to the hunt, y'know?" He stretches, pops his joints. "Sitting here with these books any longer is going to give us allergies, not answers."

"Dean, you either have allergies or you don't," Sam says, frowning over a dusty tome. He's kneading his left hand furiously, the muscles around his eyes twitching. "We've found so much; what's to say there isn't more?"

Castiel knows they don't have to bother; he knows there are no more answers in Bobby's vast library just as surely he knows the truth of every moment of history that's recorded in the books. But he doesn't dare tell them: the closer they get to the secrets behind Purgatory and the Mother, the less he can—the less he can keep them safe, the less they can... aid him in his war.

The less he can draw upon the sweet power of Sam's soul.

So unique—stripped of all the trappings that should've weighed it down, that should've tainted it with more than a century of the devil's torture, it is nothing but power. Those memories are behind a wall that Castiel does not dare to touch again, even as he can hear it cracking (the sounds like Sam's ribs breaking as his hand repeatedly plunges into Sam's chest) in that quiet, sparse room.

Sam gives up on the book; he grips his left hand so tight his knuckles turn white and holds it to his chest.

Castiel frowns. "What's wrong with your hand, Sam?"

Dean speaks before Sam can answer. "He says it's been hurting for the last week – he says he can't even move it." He leans forward, and there's something about the set of his eyes that sparks unease in Castiel. "The doctors couldn't find anything wrong – y'know, aside from the obvious."

Sam grits his teeth and glares at his brother. "Dean, I can speak for myself—"

"You want me to see if I can fix it."

And just like that, Dean's face relaxes in an easy smile. Castiel feels himself lighten too, and half hates himself for feeling that way. "Yeah. Yeah, Cas."

Castiel nods and reaches toward Sam—

(and he can see his soul straining around the edges, like that big body is too small a frame to hold its magnificence, and all Castiel needs to do is touch and he can)

—and pries his good hand away. Castiel grips Sam's limp left hand and feels the emptiness and the pain; he thinks of the room and Sam's soul unravelling from the tips of his fingers like ribbons uncoiling, and understands what's happened.

"I'm sorry," he says, "I do not know what this is. I can't fix it."

Dean's face fills with fury while Sam's closes in black resignation. Castiel lets go of his hand and disappears.

The next time he visits Sam, he's bleeding grace and blood on the floor; Raphael's dealt him a near-fatal blow and he's barely escaped with his life. It takes but seconds to ensure that Dean is sleeping too soundly to wake up to (his brother's screams) any noise for some time.

Sam wakes up as Castiel clamps a hand over his mouth. He stares in confusion before beginning to squirm and struggle; by the time Sam's protestations turn into screams, Castiel is already in the room.

The next day, Sam collapses, his left leg numb and unable to hold his weight.

This time, Castiel does not answer Dean's prayers.

"You're drunk on this power, brother," Balthazar tells him, but there is no accusation in his eyes or tone; only excitement. "You're stronger than ever before."

And there – beyond the humour and jaded speech, lies naked devotion. Castiel considers, for the briefest of moments, sharing the sheer purity of power that Sam's untainted soul gives him. After all, Balthazar has—has been loyal, has—no. No. Sam is his. Sam will... help him till he's won the war, and Castiel is winning the war.

He is.

When Castiel next meets Dean, the hunter is holding his brother in his arms, crying. He's seen Dean's darkest despairs often enough to be able to recognise its taste in the air; it hangs now, thick and cloying and black. "Dean," he says. "Sam's—"

"Blind," Dean finishes, red eyes snapping toward him with startling ferocity. "And not only that, he can't hear or speak, either. And for a moment there, I wasn't even sure if he was—" His breath hitches.

"Dean, I'm sorry," Castiel says, and he means it. "If I could do something, you know I would."

Dean nods, his jaw working. "Yeah. Yeah, I know you would, Cas. Except—" He glares at him. "I know you're responsible for this."

Castiel supposes he shouldn't be surprised; self-recriminatory, perhaps, for having underestimated Dean Winchester, even after everything. "And why do you say that?"

Dean snorts. "You think I'm stupid, Cas? Every night I get a call from Bobby asking me on your behalf about where we are, Sam—Sam loses something the next morning." He lays his brother gently back on the bed and walks toward Castiel slowly, deliberately. "I don't care what kind of shit you've got on your plate, Cas, or how much you've helped us – you cannot do this to my brother."

Dean's fury is a wonder to behold. "Dean, even if I try to explain, you will not understand."

"Understand?" Dean barks a bitter laugh. "I don't think I need to understand." And there – Castiel notes Dean's hand slipping into his jacket pocket for his lighter, and realises he's standing in a ring of holy fire. Faster than Dean can blink, he steps out, and with a flick of his hand, sends Dean hurtling to the opposite corner of the room. Dean impacts with wall-shaking force and slumps to the floor, unconscious.

He turns to Sam, who is shifting about helplessly on the bed, jaw working to form words he cannot articulate. Castiel can hear him, though: Sam's whole conscious is now nothing but panic and helpless fury and DeanDeanDEAN—

Castiel places a hand on Sam's chest. Past all the imperfections is what he really needs from Sam, and his hand plunges into the hunter's chest – much easier now, like plunging his hand in still water.

In the room, Sam still waits in the chair. The face is blank, with no eyes or lips or ears; both legs are gone and only arm remains, but there is still so much it is scarcely believable. The gossamer thread connects them once again as Sam continues to unravel.

"I will win this war," Castiel says.

Finis