Disclaimer: So not mine.
Mycroft rubbed at his right wrist. The demon's mark was well hidden by his cuffs, but no matter what he told himself it always felt like it was burning slightly.
Looking over the video feed from the hidden cameras in Sherlock and the demon's- call it John Watson, he reminded himself, we can't have Sherlock getting too suspicious of why you don't like his new 'friend'- flat, Mycroft let himself relax a little.
His brother was safe- at least for the moment- and was being well taken care of. He was even eating without coercion!
When the demon's eyes unerringly met his through the screen and gave him a smirk, he decided it was time to retire for the night.
As he crawled into bed he again thought over that night three weeks ago:
The demon appeared to him as promised and Mycroft handed it a thick stack of papers.
"Not the subtle type, are you?" It teased.
"Not when it comes to my brother's safety." The tall ginger human assured the desert uniform wearing demon. With a smirk the creäture settled down to read the contract he had been presented with.
"Protection of your brother until his natural demise. Non retaliation on yourself until and unless Sherlock dies of natural causes before you. No killing those in close contact with either your brother or yourself. No telling your brother about my true nature or the nature of my contract to you." Smiling at that last bit the demon continued to look over the presented contract.
"I can have your soul upon your natural death, or your death at the hands of your enemies. Conspiring with your enemies- or anyone else- to cause your untimely death with negate the contract and I get nothing. The contract extends beyond your death, and I will continue to look after Sherlock until his natural demise." The demon spoke softly to himself as he read through the stack of papers.
"Everything seems to be in order. I only have one small change to make." The lazy ease of the demon was not shared by the tense Mycroft.
"And what would that be?" He asked cautiously.
"You have in here that I am not allowed to consume souls while contracted to you. That just won't do. Unlike other demons I don't choose to starve myself between meals." Looking Mycroft up and down, the demon tried not to laugh.
"You can understand the importance of a regular diet, can't you?" At Mycroft's scowl the demon finally did laugh.
"Souls like yours are a rare treat for me, otherwise I mostly feast on the common herd." Taking a piece of paper and pen from the large desk in the corner the demon wrote out a replacement page for the one he found objectionable.
"This should cover all your bases that you took such pains to be explicit about presenting, and will also allow me to hunt for my regular fare." Handing the page to the genius he settled back into the chair he had taken as his own the night before.
Taking several minutes- he wanted to be absolutely sure the demon hadn't made itself a loophole of some kind- Mycroft finally agreed to the terms the creäture had presented.
"Fine. Then we have a deal? My soul in exchange for Sherlock's long life and safety?"
"Of course. Your soul in exchange for everything you put in this contract." Not knowing if he was being tricked or not with the demon's choice of wording, Mycroft held his hand out to seal the deal- after all, he had been the one to write that contract.
Instead of excepting the offered hand, the demon undid the cuffs of the expensive suit and pushed the sleeve up to reveal an unblemished wrist and forearm.
"A proper demonic contract requires a seal." So saying, the demon pressed their wrists together.
Trying to repress a scream at the sudden burning pain, Mycroft gasped when it was suddenly gone. Looking down at his wrist he saw a circular tattoo, it matched the one now adorning the demon's wrist.
"Now I must be off." It said cheerfully.
"You were fairly vague on how I was to approach your brother, so I do need to figure out the best way to do so." Turning away with a smirk, it headed back towards the fireplace to make its exit.
"You don't even know where he is." Waving his concerns away the demon stepped into the fire and was gone.
Two days after he was released from the hospital Sherlock Holmes was introduced to Dr. John Watson.