John was out and they hadn't had a case in nearly three weeks. Sherlock was seriously considering arranging a murder so he could stop being so damn bored. The cold cases Lestrade had sent him weren't working anymore, they were all too simple. Mrs. Hudson was never exactly the pinnacle of intelligent conversation, and besides, she was visiting Mrs. Turner, and he didn't have any experiments he could line up easily. If not for John, he likely would have shot up by now.
So naturally, when his phone buzzed, he leapt on it.
Bored, gorgeous? How about hide-and-seek?
-JM
He couldn't help but grin, heart racing. Now there was the kind of thrill he'd been craving.
I need an address.
-SH
It came quickly, somewhere abandoned from what he remembered of maps. He snatched John's gun out of its place in his dresser (top left drawer, obvious) before he dashed out for a cab, looking for all the world like a giddy schoolboy.
There was no sign of Moriarty-or, for that matter, any human being-when he arrived at the house, a decrepit establishment. The dust patterns told him nobody had come in for some time, and for a moment he wondered if he'd come to the right place, but the text had said hide-and-seek, after all. For the time being he explored the place, memorizing the structure and the shadows. There was nothing extraordinary to it until one of the shadows moved. There was no source to the motion, unexplainable. Shadows didn't move by themselves.
A fear he didn't understand crawled up his spine. This thing was no more than a whisp of smoke, practically transparent, and yet there was a darkness behind it that made his blood freeze in his veins. It pulsed, swirling in front of him as if it were hesitating. Cautiously, he took a small step forward. The thing froze, not even twitching, then floated a few inches closer.
"What are you?"
It glowed scarlet, then a voice scarcely more than a hiss came into Sherlock's conciousness. It did not speak audibly, rather it was as if a thought not his own had flitted across Sherlock's brain. The thought was but a single word, a whisper- Demon.
He staggered backwards. "Get out of my head."
The demon laughed, which was a thousand times worse. The gnarled sound rang through every thought. You have a fascinating mind. I will enjoy watching it collapse. It lunged forward before Sherlock had a chance to react, aiming straight for his chest. Pain ripped through him, spreading outward like a blaze. Was he screaming? He must've been; agony like this couldn't be endured in silence, but the roaring in his ears overtook everything else.
Quickly as it began, the pain ceased, replaced by an icy numbness that seeped through his veins as he lay gasping for oxygen. Do not try to fight.
It sounded so calm. Infuriatingly so.
"This is Moriarty, isn't it? You don't exist." Sherlock hissed through his teeth.
You don't believe in me. That fact does not diminish or invalidate my existence. I am every bit as real as you, and no mortal, not even dear Moriarty, could replicate this-
Another wave of pain, sickening in its intensity, washed over him.
Do you see?
"Y-yes." he forced out. "What do you want with me?"
You are not at liberty to ask questions. But I like an intellectual, they go down the hardest. And you...I can already see that you will not be an easy break. Good.
"What do you mean by break?"
I am here to prove to you just how insane you really are. You might learn something. And as I've shown you, it is unwise to fight me when I have made up my mind. The first thing you will learn is just how little control you have.
The liquid ice in Sherlock's blood seemed to suddenly solidify. There was a moment of dizziness, and, suddenly, he was on his feet. "Alright, you can make me move where you like. Now what?"
Now we go home, Sherlock. Don't you want to see John?
John. No.
You know what happens if you fight me.
"I'll go where you like, just please leave John out of this."
I shall consider it. For now, you will comply or I can guarantee dear John's death by your hands.
Sherlock swallowed. "Fine."