It was difficult to describe at first, just that something was wrong, something was not quite John.

"Would you like a cup of tea?" Sherlock tried one morning, testing the waters.

"Yes, thanks," John but Not John replied, smiling in a way that was similar but not, content but off kilter, damaged and false. Just… false. Sherlock frowned. He never made tea, and yet there'd been no raise of an eyebrow, no curious smirk, no suspiciousness at all. Sherlock frowned deeper, bordering on a pout. Because he didn't understand. And he hated not understanding.

"You seem different," Sherlock kept on, aiming for blunt, but this time John did raise an eyebrow at him, and even that was wrong.

"Different how?"

"I don't know," Sherlock growled, slamming the kettle onto the stove with more force than he'd intended. Also wrong. "You're just… Not acting yourself. Is something-" Sherlock cut himself off, going silent at the sight of that minuscule almost unnoticeable shift of John's lips, the smirk that wasn't quite a smirk but more like a grimace. A smirk that didn't belong to him. A smirk he'd never seen in seven months two weeks and three days on John's face. This wasn't John. He didn't know how that was possible, don't didn't how to prove it, didn't even know if he was right-which was all manner of frustrating-but he was certain, somehow, deep down. This wasn't John. This wasn't his flatmate, his friend, his John. This was something other, something wrong.

And when you eliminate the impossible, whatever remains, however improbable, must be the truth. He just had to figure out how to prove it.

"So you think John's… possessed." Lestrade repeated for the fifth time since Sherlock had barged into his office and proclaimed as such. Which was why Sherlock couldn't hold back the practical yell of annoyance that escaped him.

"I told you, the man who's living in my flat right now isn't John." Sherlock tried again. "I don't know how and I don't know why, but something's changed him."

"You've been living together for how long?"

"Seventh months, two weeks, five days," Sherlock replied without hesitation, Lestrade nodding slowly.

"Are you sure he isn't just getting comfortable with you? Showing you who he really-"

"I know who he really is!" Sherlock yelled. Lestrade put his hands up in defeat, opening is mouth to more than likely attempt to calm Sherlock down, but Sherlock was already halfway out the door. "I know who he is," he repeated. "And I'm getting him back. With or without your help."

"Didn't take you long to figure it out, did it?" John's voice met him at the foot of the stairs, John's body leaning back into the couch cushions, John's feet propping themselves up on the sitting room table once Sherlock was inside, but none of it was John's doing. None of it was John. "They said you were good, but I just had to see for myself," the John but Not John stretched lazily, throwing his hands behind his head in a mockery of comfort, like it was emulating it but not quite understanding it. "So what are you going to do now, Mr. Consulting Detective? Torture me? Brush up on your Latin? I found the books, you know. Hidden under your bed like old porn mags. But I suppose that makes sense. The good doctor was the closest thing you'd ever had to a fantasy…" He glanced at Sherlock for the first time, out of the corner of his eye. "Am I right?"

"So you're what then? A demon?" Sherlock said calmly, hanging up his coat and taking a seat in front of the Not John, crossing his legs and bringing his palms together, tips of his fingers resting against his lips. "I've heard stories of your kind, but never come across proof. I must admit, it was surprisingly difficult to find data I could trust."

"Oh, I wouldn't trust any of it," Not John smirked. All wrong. All wrong. "But that's just me." As if to accentuate the fact, he closed his eyes, a fraction of a second, though too long to be a blink, and reopened them to reveal solid black scleras, the full of his eye tainted by it, all color drowned out by it. Sherlock had to stifle a gasp, though he couldn't contain his frown. "So how about we get to know each other better?"

"No thanks," Sherlock sighed, feigning boredom. "I'd rather you just leave John alone." He narrowed his eyes at the Not John. "Before I'm forced to make you."

"Make me…" Not John parroted as if the words didn't make sense, then a devious, inhumanly twisted grin stretched across his face. "Make me? MAKE ME?!" He was laughing then, the sound shrill and ugly and damaging to the ears, a cruel rendition of one of the many Sherlock had put away in his Mind Palace behind a door marked "John's Laughs." It was closest to the one he offered when Sherlock was unfamiliar with something he deemed common sense, but not close at all. Just not. "What are you going to do, Sherlock Holmes? Deduce me? Just try figuring out shit while I'm buried soul deep in your pathetic flatmate. Do you even realize why they let me possess this broken down, emotionally crippled excuse for a solider in the first place? Do you know how much he's worth?" Not John was on his feet now. "How much you're worth?" He took a step forward, resting both hands on the back of Sherlock's chair. "You two… are my meal ticket."

Sherlock watched those black eyes in silence, studying them, waiting for Not John to explain, but it was taking too long, egging him on with a silence of its own. So Sherlock leaned back, crossing his arms over his chest. "A demon of the lower levels in your pecking order then. Interesting. Looking to move up the ranks, perhaps? What could some manner of demon hierarchy want with John and me?"

Not John frowned, letting his head drop just slightly, his shoulders roll back. A groan escaped him that made something in Sherlock's stomach clench. "Oh, you should see inside his head, Sherlock," Not John practically purred, licking his lips. John's lips. "You should see all the things he wants to do to you." This time, when he looked up, his eyes were back to normal, Not John leering at him with John's eyes. It was almost too much, infuriating, painful. "He would never admit it of course. You're not a couple, yeah? He's not gay? And you…" He paused, blinking slowly, searching. Searching John's mind, his memories, tampering with what was John's, invading what was their's. "You're married to your work." Not John scoffed. "But I don't think that's true." Not John got to his feet, walking forward in a way that reminded Sherlock of a snake, slithering towards him and eyeing him hungrily. "How about we find out how true that is, shall we?"

Not John was straddling his lap before Sherlock had a chance to process what the demon meant, Sherlock's shoulder's stiffening as he sat back out of reflex, watching Not John dip his head down to plant a kiss along the line of his neck.

Stop, Sherlock wanted to say, because this wasn't John, this wasn't John, this wasn't John, but it was John's body and John's voice and words were being whispered in his ear and for a second, he didn't know how to speak. But Not John did.

"He can see all of this, you know." Gritty, lingering whispers to match nips of teeth and presses of tongue against shivering, damnably responsive skin. Not John rocked his hips, just once, an electric current rushing up Sherlock's spine. "He can feeleverything." He rocked his hips again, lips tracing Sherlock's jaw, forming words with John's lips against the corner of his mouth. "He wants you so bad, Sherlock. He's crying out, in his head, begging for it. You wanna hear him?"

Sherlock meant to say no, meant to lie, but the second of hesitation, the lack of immediate response, was apparently enough.

Not John closed his eyes, let his head fall back and when he returned it was like the opening of floodgates in Sherlock's chest. Because those eyes belonged to John, the right John, his John. He'd know them anywhere.

"Fuck, Sherlock," John groaned, rocking his hips again. Sherlock bit his lip, trying and failing not to let the beautiful sounds and impossible sights affect him. This was only a temporary respite. This was a taunt, a tease, and soon his John would be locked away under the gaze of Not John once more. Sherlock had to figure out a way to keep that from happening, but dammit all if having John wanton above him wasn't just the right amount of distracting.

"J-John," Sherlock tried, licking his lips and steading John on his lap. John seemed torn between what his body wanted and what his mind needed, shaking his head in earnest confusion, a tangible display of lost clarity.

Though, true to form, it wasn't long before the words leaving at a whisper from between John's lips shifted from, "God yes, please more," to, "Help me, Sherlock, end it please." And that was all Sherlock needed.

The kiss was tainted and damaged and bittersweet but still perfect somehow, because underneath the stretch of possessed skin was the beating of a familiar heart. John was his and their kiss was true and when this was all over, Sherlock planned to kiss him again and again. He could almost thank this damnable creature for opening those doors, but it seemed a bit not good. Instead, Sherlock nearly praised the lord, until he realized the irony. If God did exist, it was unlikely he had a hand in this at all. Regardless, John's lips were soft against his own, his body pliant, and when Sherlock placed a hand to John's cheek, his face seemed to tilt into it, desperate and sweet and hopeful. Which was when Sherlock retrieved the knife from under the chair and stabbed John in the side, just beneath his ribs, avoiding anything vital.

Light flashed orange and blinding in John's eyes, a desperate heat that burst through every orifice of John's face as if trying to escape. Sherlock cringed, the feel of John's skin giving way beneath the knife causing a painful, sickening lurch in his stomach to deal with later. For now, this was the only way, the best way. This, if handled well and quickly, left Not John behind for his John to return. It just boiled down to whether or not he could patch him up quick enough. He was by no means a doctor, but he'd been planning this for months. He'd readied himself.

Getting his hands on the blade had been tricky compared to the books filled with ancient mythos and Latin texts. It had taken every ounce of Mycroft's not insubstantial connections, ultimately leading to a duo of brothers in America hardly willing to part with it. But at the promise to return the precious item, Sherlock had managed. And now John, the real John, was rightfully present once again. And dying in his arms.

"Not how I would have handled it, but I suppose it can't be helped," John coughed, the presence within him apparently long dead. Or perhaps just transferred, it was hard to tell; the brothers hadn't really explained much past the initial stabbing. Sherlock cleared his throat, grabbing his mobile from his pocket and sending a quick text.

Contact the Winchesters. Tell them to retrieve the knife in person. –SH

The reply came within seconds.

They'll be on the next plane. An ambulance is on its way. Please do remember to stick to the story. My team can only cover up so much. –MH

Sherlock opted out of his own reply, chucking the phone to the floor instead with a poignant roll of the eyes. He had more important things to deal with than feigning break ins.

Carefully, Sherlock lowered John to the floor, heart tugging harshly with each wince of John's eyes, each hiss of pained breath that escaped between John's kiss bruised lips. Sherlock had done that. After all this time, he'd finally been allowed to claim those lips, though the situation hadn't been ideal. And the knife still sticking out of John's side seemed like enough proof against it happening again soon. Sherlock had done that too. Feeling John's skin giving way to the blade would remain forever undeleted, a reminder of what he'd almost lost. And, as it turned out, so would the feel and sound of removing the blade from that same patch of crimson soaked skin.

John lurched beneath him for a moment before falling limp, breathing ragged and strained. Sherlock let the knife fall to the floor, grabbing for the clean washcloth that had also been stocked beneath the chair and applying the right amount of pressure to the bleeding wound.

"Sherlock," John tried to speak but his name sounded half whispered, half choked, so Sherlock pressed a bit harder, shaking his head.

"Not now, John. We'll deal with the data once you've stabilized." Sherlock replied to the probably steadily growing list of John's concerns, but the look in John's eyes only worsened, brows furrowed in something other than pain.

"No, Sherlock," John swallowed. "I saw in its head." He didn't need to clarify.

"Whatever it wants with us, we'll manage," Sherlock tried to brush it off, though he couldn't deny the rush of a new mystery trying to force its way past Sherlock's concern for John. He wouldn't let it, not that it stood much of a chance. "Mycroft has two of the most knowledgeable members of their field heading to London as we speak. They'll know how we should-"

"Not us," John cut him off, closing his eyes for a moment. Sherlock felt his first trace of genuine panic begin to set in. When John opened them again, a small rush of relief settled in its place, though the look in John's eyes was distant. "They don't want us, Sherlock. They want you."

This time when John closed his eyes, they didn't open again until two days later. And Sherlock didn't leave John's side until they did.

"You're not going alone," John frowned, the look losing some of its strength as John fought to sit up in his hospital bed and failed.

"I won't be alone," Sherlock offered, placing a hand to John's shoulder and forcing him back against the bed. "The Winchesters are proficient hunters. They've offered to accompany me while I-"

"While you what?" John crossed his arms of his chest, his exhale filled with no small amount of disdain. "Go off chasing a group of mythological creatures that want you dead? This is ridiculous!" Before Sherlock could argue further, John ran a hand over his face and continued. "I know their plan, Sherlock. I know what they want from you, and you're not prepared to fight them. No one is."

"The Winchesters have beaten this 'King of Hell' before," Sherlock tried, but John wasn't having it.

"They know your weakness, Sherlock. And if I'm not there to remind you of it, not even the Winchesters stand a chance."

"What weakness," Sherlock scoffed. "I have no-"

"That. Right there," John pointed. "There it is. You think you're invincible, that you're clever enough to go undefeated forever. But the thing is, you're not going up against humans anymore, Sherlock. These things don't think like we do. They don't follow the same rules, if they follow any. You think you can beat them like you beat everyone else, but they're counting on that. They're hoping for it." John wasn't looking at him anymore, his mouth drawn in a tight line that matched the creases in his forehead. "You saved my life Sherlock, I get that. If either of us owes the other right now, it's me. But please. Just wait until I can come with you."

Sherlock knew as much of the plan as John's possession had granted them. He knew what they wanted from him and what they were willing to do to get it. He'd already lost John once. He wasn't letting it happen again. So, with every ounce of his once substantial acting ability, he forced down the pain in his chest and the lump in his throat and said, "You'd only slow us down."

John might have looked at him then, might have been shocked and betrayed and hurt, but Sherlock didn't wait around to see, hurrying out of the hospital room with a barely contained farewell he knew John couldn't hear.

It was for the best.

If this 'King of Hell' wanted a deal that badly, he'd give him one. And if it didn't end well, at least Sherlock would die knowing that John didn't have to. At least he would die having known the feel of John's lips against his own.