A/N:I wrote this for a friend on tumblr and the inspiration was set by this gif: post/29177093631

"You know I like this new pet of yours. He's touchingly loyal." A pause. A rustle of movement. "He screams for you, you know." The familiar sound of the daily paper being opened and manhandled. "Endlessly." John's voice added.

Sherlock paced, hand at his mouth, hair disheveled, nerves skyrocketing, heart racing.

How could that thing, that demon, just sit there, reading the paper and reminding Sherlock of how tortured John was, trapped inside his own body, so flippantly as if it didn't even matter? But, of course it didn't matter. Not for a demon. Not for an impossibly existent creature that held no emotions, didn't care, didn't love, didn't hope or dream or cry, didn't feel.

Though, just saying that inside Sherlock's head, he couldn't help but remember all those times that everyone said the exact same thing to him. He'd been called callous, an annoying dick, a right foul git, and any other name or combination of such in the book. At the time, he'd brushed it off. It didn't matter what people thought of him, they were all inferior anyway and their opinions didn't change the facts, the data.

But now, he kept recalling those names, those descriptions, using them to describe the thing sitting in the chair, wearing John like he was just another piece of clothing.

For it was John. John's hair, teeth, face, hands, legs, body. John's everything. Except it wasn't.

Not completely.

It was John's look alright, but it wasn't, in fact, John himself. The thing sitting in the chair now, reading over the newspaper, eyebrows furrowed just a bit as he concentrated—"You humans are just so funny, aren't you?"—on an article didn't have John's mirth, the emotions that plagued the man, the John's lovely smile or the glint of joy in those eyes, which, more often than not, shone black, the only physical indication that it wasn't John.

Sherlock threw himself onto the couch, head resting in his hands as his teeth worked his bottom lip, his mind working through the impossible.

But it was possible, wasn't it? He had the evidence right in front of him! Everyone had seen it, too, eventually. Lestrade, Molly, Anderson, Mrs. Hudson, Mycroft, Sally, Sarah and others. They'd seen its eyes, too, so Sherlock knew he wasn't going mad. Impossibly mad.

And, yet, he was.

Day after day, that demon sat there, stood there, followed Sherlock around just like John would, ate and slept and cooked and read in the flat. It's been here for a while now, Sherlock supposes, if he looks at the data again.

Six weeks ago, John and Sherlock had been on a case involving something that doesn't matter now. John, of course, had gotten attacked and Sherlock, uncontrollably angry at this, dashed off to take care of the one responsible. But when he came back to check on John, he found that he wasn't there on the floor like Sherlock had seen with blood dripping from his nose, pouring out of his side where the knife wound was. Instead, he was standing by a window, fixing his jumper, seemingly perfectly fine. Sherlock, of course, was worried and had taken him to the hospital only for the doctors to examine him and find nothing. Not a broken nose or a knife wound. That was possibly the first major indication that John was not the same.

There were other signs, along the way.

Like Sherlock, John wouldn't sleep for days. Though, he usually did that anyway if Sherlock needed him to. If they were working on a case that needed constant attention. John would have gladly volunteered for staying up for days at a time with Sherlock to solve the case. But these times were different. Mainly because no such case had been involved and John didn't seem as if he was being affected by lack of sleep. When Sherlock had questioned him about it, had asked him if he was alright and why he wasn't currently keeling over in exhaustion, John had only shrugged it off and said that it was nothing, he was fine, just had too much energy to sleep.

But Sherlock knew he was lying. He just didn't know why.

And then there were smaller things. John didn't ask if Sherlock would like to come out with him, to get out of the flat for a bit because it simply wasn't healthy to stay inside all the time. He never ordered Sherlock to eat something. John stopped walking around in his bathrobe after a shower. He never went into his bedroom. Hardly ate or drank anything. And because of that he hardly ever went to the loo. John stopped talking. Sure, the man was typically quiet—after all, Sherlock didn't really make a good conversation partner—to begin with, only ever succeeding in conversation with Sherlock every once in a while in a given day. But this was eerily quiet. It was almost as if he made no noise whatsoever. He'd actually managed to sneak up on Sherlock quite a few times, which in itself was worrisome. But Sherlock had brushed it off as himself just not paying attention—hell, he had forgotten himself that John wasn't there sometimes when he was rattling off on something.

And then time went on. And it was painfully clear that something was terribly wrong with his doctor, his blogger, his flatmate, his friend, his John Watson.

He just didn't know what it was. Not until a few weeks ago. Not until that thing finally blinked open John's eyes after a nap in its—his—habitual armchair and black eyes looked up at Sherlock to where the man was looming over him.

Needless to say that Sherlock had been startled, had even jumped back quite a few feet and aimed a gun at the thing's—John's—head.

"You're honestly going to shoot your only friend?" It had said, using John's voice, but slowly standing to its feet and its lips quirking into a victorious smirk as Sherlock lowered the firearm. "Thought so. Also, it'd be a waste. I'm quite fond of him already."

Sherlock didn't speak. Not at first. What was he supposed to say? This wasn't some sort of case, some familiar territory that he could easily navigate with his mind! He mind listed off supernatural beings, myths and legends, and he landed on one that he had heard of in passing. But it didn't make sense! It couldn't make sense! Because those things didn't exist in the real world!

"Oh but we do, Sherlock. We do exist." It had whispered into the still silence.

Had it read his thoughts?

"No, just reading your body language, love."

"Don't call me that! Don't you dare call me that using his voice!" Sherlock hissed out, face heated with a burning passion to slit its throat. Even that throat didn't belong to John, that is.

And that had been their first actual encounter. With speech and everything.

The demon had blinked and his black eyes were gone, replaced with John's loving ones, the ones that said so much but were now empty. Except….if Sherlock looked closely enough, he could see pain. Deep horror in those eyes. John knew. He might not be here anymore, but he knew what was happening. What that thing was inside of him. But he was trapped.

The knowledge of what was happening, of what that thing was, hit Sherlock so fiercely he couldn't breathe. His heart was beating too fast in his ribcage and within seconds he was on the floor. Vaguely, he could hear that thing, chuckle derisively, "And I thought he was going to be much stronger than that."

It didn't take long for Sherlock to go mad after that.

It didn't take long for that Demon to completely destroy any trace of John.

It didn't take long for Sherlock to lose all hope that John would come back to him.

It isn't taking long for Sherlock to shed the first of many tears.

And then John's voice rang throughout the silence again, the sound of his rising from the armchair, placing the newspaper in his spot before he walked over slowly to Sherlock.

Pale, long fingers gripped Sherlock's jaw in a bruising grip, forcing him to look up at the thing's—John's—eyes. "Oh, do pull yourself together. At least you know he's still here. For a bit." And it was smiling, shining those brilliantly white teeth at Sherlock as if this entire thing was funny. "I know what he means to you. And I know what you mean to him." He leaned in close, whispering, his breath ghosting over Sherlock's lips as he continued, the scent of mint toothpaste assaulting Sherlock's nostrils. "It's still his body, though. There's still his primal needs and wants inside of here and I wouldn't mind having a go at you." He said, his voice unwavering in its slow whisper, but there was a threat there.

A threat to do what, Sherlock wasn't sure. Rape, certainly. But not in the near future. And there was another threat just below that one. To kill him? But he would have done that by now if that were the case.

Did it only want to torture him, then? Drive him slowly insane, to the point where it's irreversible and someone ends up bloodied and dead in the flat?

Before Sherlock could react, could question it, do anything but stare at the man that used to be his John Watson, it was pulling away, sighing and straightening up. "We'll have some fun, soon, yeah? Oh, yes…definitely soon, love."

As it moved to close and lock the door, Sherlock, for all his brilliance and wit, couldn't do a single thing, couldn't talk himself out of this situation, couldn't find the need to want to do anything about it.

Because without John…

Because with that thing…

…he might as well die already.