Fully absorbed in his mind palace, Sherlock was stretched out on the couch in Baker Street, positioned in his usual thinking pose. Due to his mind being completely occupied with the case Lestrade had told him of, he didn't hear the footsteps that approached in an uncharacteristically quiet way, avoiding the creaking of one of the stairs. When Sherlock sensed another presence in the room, he snapped one eye open and saw John sat in his chair. The case could wait.
"Sherlock," John greeted coldly with a nod of his head.
"John." He sat up and waited for the conversation to resume, not wanting to rush into explanations if John wasn't ready, because John's emotions were very important to him. Silence had filled every dent and corner of the room, threatening to make the situation even more awkward than it already was.
"So how did you manage it then?" The question was straightforward and blunt, so much so that Sherlock was stunned for a second before answering.
"Moriarty wanted to kill you, Mrs Hudson, Lestrade..."
"So you had to die instead," John continued for him. "I'm amazed you managed to keep quiet for this long."
"You understand that if I had any other way to save you, I would have." Sherlock remembered the moment when Moriarty had shot himself after the brief conversation of not needing to die if he had him.
"Oh Sherlock," John's voice was calm. "This is too fun."
Sherlock raised an eyebrow and narrowed his eyes. People weren't meant to respond like that to the situation.
John began to laugh to himself. "I'm sorry, I can't keep this up anymore." Within a short second, John blinked and suddenly was not John anymore. His friendly, comforting eyes became completely black.
"If only you had an idea of how much pain John suffered while you were gone."
"You're not John. Who are you?" Sherlock grabbed the person by the shoulders. "What are you?"
"Careful now," they responded with a grin. "John's still here, he's just not in control of his own body right now. He's scared."
Sherlock tightened his hands on the shoulders and his face became more threatening.
"Calm down or I'll break his bones. I'll torture him, because whatever I do to myself, he will feel the pain of. Not to mention I can read his thoughts."
"Who are you?" Sherlock repeated his question as he let go but didn't step away.
"You can stick to calling me John, since I might stick around a while. You're interesting for a human. John thinks the world of you."
Sherlock felt his heart sink knowing John was trapped inside his own body, finding out his friend was alive for so long secretly and discovering that in such a horrible way.
"What are you?"
"Ever heard of demons?" Fake-John walked towards the kitchen and flicked the button on the kettle.
"I was never one to believe in things I cannot observe." Sherlock sat down in his chair.
"Tea or coffee?" they smiled in response. "Never mind, John said you like coffee."
Sherlock's eyes narrowed again as he watched Fake-John making the coffee correctly. They would only know that from what John was thinking.
"Why did you possess him?" Sherlock asked after a few minutes of silently drinking coffee with the demon.
Fake-John hesitated, drank another sip of their tea then answered with a straight face. "He doesn't want me to tell you."
"Like that would stop you."
"Don't pretend you know me, Mr Holmes. We have only just met."
And for the first time in a long while, Sherlock really couldn't deduce anything about the demon. Nothing of importance, anyway.
"How come you told me who you were so quickly?" he continued.
"I thought it would be more fun to see your response to not being able to save your friend." They remarked with a small giggle. "It's funnier now I know that you pretending to die just to save him last time."
Sherlock stared at fake-John as they made their next comment with another small laugh. "John doesn't like me saying things like that. He's awfully defensive of you, Sherlock."
"How long do you plan on staying and what will it take for you to leave?"
The demon looked disappointed by his question. "Trying to get rid of me so soon? I expected you'd want to find as much as you can about me. But to answer your question, I'm going to stay for as long as I please and there's nothing you can do to make me leave. Sorry!"
"Leave John alone and possess me instead," Sherlock practically demanded. "You'll see all of the observations I see, understand the deductions I make."
Fake-John seemed to consider this for a minute however shook their head slowly. "No... Maybe later."
"Fine." Sherlock snapped. "I have a case to solve."
Sherlock stood from his chair, placed the empty coffee mug on the table (after stepping across it, of course), then recommenced his earlier thoughts from the comfort of the couch.
Before fading away to his mind palace, Sherlock heard Fake-John say something in an almost whisper.
"John says you did this a lot."

"Fingernails!" Sherlock yelled, his hands thrown into the air and eyes wide open. "John, call Lestrade."
The realisation that John was not actually John hit him after he made the instruction, and he looked to see the man sat in his chair staring at him with a small smile growing on his face.
"Sure." He answered, picking up the phone and dialling the number.
Sherlock raised an eyebrow but put his coat and scarf on.
"Lestrade," Fake-John greeted on the phone. "Yes, it's John."
There was a moment of silence as he was listening to the reply before interrupting. "Yeah, Sherlock solved it, something about fingernails apparently. We'll be over in a few minutes."
Fake-John hung up the phone and threw a jacket on quickly before looking at Sherlock expectantly.
"Well? Come on."
"Why do you want to come with me?" Sherlock questioned.
"I'm intrigued." Fake-John shrugged his shoulders. "Now let's go, I'd like to see what this case is."
The pair left Baker street quickly and were in a taxi on the way to meet Lestrade when Sherlock caught Fake-John watching him.
"What is it?"
"I have to admit," he explained. "I didn't expect you to be taking all of this in your stride. I've admitted that I am possessing your friend that you haven't seen for ages and yet you're quite fine with that."
"It's interesting." Sherlock looked out of the window and avoided further conversation.
"Quite."

Sherlock almost ran to Lestrade's office, bouncing up the stairs two at a time with Fake-John following close behind him.
"Sherlock," Lestrade smiled as he walked in. Sherlock quickly deduced that the smile was a genuinely happy to see him, but he had tired eyes and his skin was paler than usual. There was a fresh cup of coffee on the desk but dry marks around the cup so he had already drank at least one within the past hour. Lestrade was lacking in sleep, but not due to any family or work related matters. No, this was because he was still trying to get his head around the return of his dead friend.
"John, it's good to see you. It's been a while," he greeted.
"And you," Fake-John replied with a quick grin.
"So, fingernails?"
Sherlock's eyes flashed from Fake-John to Lestrade and he began to reveal the mystery behind the case. "Kara bit her fingernails, it was something she did when she was nervous or stressed, but it was only a recent habit so she knew something was coming, something was going to happen to her. Obviously it was murder, we established that, but it was pre-meditated and provoked."
"Provoked? How do you know that?" Lestrade asked with a raised eyebrow. Sherlock could feel Fake-John's eyes staring at him.
"She was addicted to gambling and ended up running into financial problems, so she had to borrow some money from wherever she could get it." He explained. "Stupidly and desperately, she picked the first person she found who happened to give her a large interest rate with a short payback time and unfortunately it cost her life. The guy you're looking for is Robert Willison."
"Thanks, Sherlock." Lestrade nodded his head and began dialling a number. "What did I do without you to solve these cases?"
Sherlock grinned before leaving the office with Fake-John who seemed quiet. They walked down the stairs to leave and eventually he spoke.
"John was right, that is quite cool."
Sherlock laughed a little bit. "I'm flattered."

A/N: So the mystery is revealed as to why John is acting differently! Little note that I'm so happy with the response I'm getting from this, and reviews will be very much appreciated, so if you've got the time to write one and something to say, please do! I'll try get the next chapter up within a week. x