This work is a conglomeration of the 30 day Monster boy challenge, which can be found on Tumblr. Each chapter is a little one-shot, a different monster every day, just to celebrate the fantastic month of October! I hope you enjoy!

Day 1: Demon

My Soul to Keep

(Rated T)

It was no fault of his own that John Watson became the target of a demon. The man had always lived a good life, the people around him, however, were a different story. He had always been on the edge of the demon's awareness, flitting around just out of sight. One soul exchange in the middle of a seedy pub changed all that

"You want your wife back." The demon sneered at the scraggly looking blonde woman. The state of her sobriety was questionable, though she appeared to be better off than anyone else in the pub. "And you are attempting to sell your soul for such a request...how dull. Unfortunately for you, your soul is worthless. Goodbye, Harriet Watson."

He turned away from the blonde, annoyed that his time had been wasted so poorly.

"Wait!" She called, and the demon rolled his eyes, turning back to her. "What about family? I have...I have a brother. He's a good bloke. A soldier..."

"He would have to be willing to agree to this pact." Sherlock responded, his voice betraying nothing. "Do you believe he would?"

"Yes. He would...that's Johnny..." Harriet nodded emphatically. "Please."

"I shall visit him." The demon replied. "But if he refuses, then it shall be your soul that shall be taken, and you will die in disgrace."

Harriet paled as the demon turned away from her, vanishing with a swirl of his long, black coat.

The last thing John Watson expected when he entered his small bedsit was company, but that was exactly what he got. A pale man with dark, inky curls was sprawled across his desk chair, looking as if he owned the place. He turned as John entered the room, pale eyes assessing his every move.

"John Watson." The man said by way of greeting.

"That's me." John steeled himself. "Now who the hell are you, and what are you doing in my bedsit?"

The pale man smirked, standing up in what John assumed was an intimidating gesture. "I am known as Sherlock, and I am here about your sister."

"Oh god, what has she done now?" John groaned, making his way to his bed and sitting down, the springs squeaking in protest. "Drank all her money away? Are you part of a...a gang or something?"

"Not quite, no." Sherlock smirked, "Something much more sinister, I'm afraid. Your sister was desperate to get her wife back."

"What do you mean, more sinister?" John asked. "What could be more sinister than a gang?"

"I am afraid your sister tried to sell her soul. It was worthless, so she offered yours in exchange." Sherlock walked towards John, towering over the sitting blond. "If you refuse, your sister's soul shall be destroyed."

John snorted, standing up. He was still significantly shorter than the man who claimed to be a demon, but he couldn't be bothered to care at the moment.

"You really want me to believe that you are a demon?" John asked, barely supressing a giggle. "That is the most ridiculous thing I have ever heard."

Sherlock's smirk widened, and John watched as his blue eyes swirled into a terrifying black, eclipsing the whites of his eyes. John's breathe caught in this throat, and in his attempted to escape, he fell backwards against the bed. The black eyed demon leaned over him once more, his grin looking wickeder than it had a few seconds earlier.

"Christ…" John swore, trying to crawl away from the imposing figure. Sherlock halted the movement with one hand, fingers curling into the soft fabric of his jumper.

"Not even close." Sherlock said, blinking so the darkness in his eyes vanished back into the pale blue that John had already become familiar with. "Nearly the opposite of Christ, actually."

"So you're a…a…"

"A demon. Yes, I've already told you this. Do keep up."

John slapped Sherlock's hand away, annoyed as the demon chuckled in response before stepping away. He sat up, rubbing absentmindedly where Sherlock's hand had been.

"So, Harry…"

"It is either you, or her." Sherlock cut him off. "Her soul will be destroyed."

"And mine?" John asked hesitantly.

Sherlock paused, clearly choosing his words carefully before responding. "Your soul will not be destroyed, however, you will not be allowed to leave my realm."

"What does that mean?"

"Essentially, you shall be my slave. In return, your sister shall get her wife back, and her soul will be saved."

"She won't go to…hell, or wherever it is your realm is?"

The demon chuckled, clearly pleased with John's reactions. "No, she will, unless she is a complete idiot, go to heaven."

"Right. Right." John nodded, running a hand through his hair. "I accept."

"Excuse me?" Sherlock frowned, looking down at the ex-soldier. "You accept?"

"Yes, I accept." John replied, striving for a calm, collected tone. He was failing miserably. "I will give you my soul in exchange for my sister's."

"You will?" Sherlock's eyebrows rose, and John felt momentarily pleased at being able to surprise the demon. "Your sister was right. How very wonderful for her, not so much for you."

"Anything to keep my sister out of trouble." John's voice was world weary. "Do I need to do something to…er…seal the deal?"

Sherlock hummed, walking over to John once again. "We shake on it."

John raised his hand, offering it to the demon who grabbed it quickly, a mischievous glint in his eyes. He pulled the shorter man closer to him, leaning down to whisper in his ear.

"By agreeing to this contract, you agree to be in my servitude until I so choose to release you of your duties. Doing so protects your sister, Harriet Watson, as well as her wife, from my wrath."

"I…I agree." John said, speaking more to Sherlock's ear than anything else.

"The contract is finished." Sherlock said, "Now we shall seal it with a kiss."

Before John had any time to react, the demon shifted, pressing his lips against his roughly, his hand clasping the back of the human's neck to keep him in place. John's hands fisted in the collar of the demon's coat, clinging to him as they vanished in a puff of black smoke.

The next morning Harriet visited John's bedsit, unsurprised to find nothing but the faint whiff of sulphur and a small note written in spidery scrawl:

He agreed

A/N: No current plans on expanding this story line, but I can always be swayed if you all so desire!