Disclaimer: This story is based on characters and situations featured in TV series "Sherlock BBC" and the series of books "Harry Potter".

Warnings: language, violence, sexual content

Chapter 1: The Past is Knocking on the Door

The streets in London were not really an appropriate place for running away. Unfortunately, John Watson had no other choice at the moment because his past was chasing him and being caught would mean only death. The autumn evening gave him some advantages, so he could hide himself in the dark, narrow alleys, but he knew that the one pursuing him was about to use every possible means to catch him. And to think, that John could have been now sitting in his comfortable chair at 221B Baker Street. It all happened because he had not been careful enough. Four years of hiding and now, because of his stupid whim, he could lose everything.

"Damn it!" John cursed as he struggled with his sleeve where his wand was being kept at the moment. To think that the years of ignoring his magical heritage would mean that he even had now trouble with getting out his wand. And in a moment when his life literary depended on it.

As he at least managed to pull his 12 inches long hawthorn wand with a dragon heartstring for its core., he sighed contently at the feeling of magic flowing through him as he held the wand in his right hand. God, how he missed this feeling. But John did not have time for leisure, he needed to act quickly. The auror was right behind and the moment he used a spell to disarm him was coming closer with every second. John was a skilled duelist but he knew that now he didn't stand a chance. Not only hasn't he used magic for nearly four years but also the auror who was after him was no one else but the Savior of the Wizarding World – Harry Potter. If Dark Lord hadn't stood a chance against the man, then John certainly didn't either.

John ran as fast as he could while trying to remember any spell that would get him out of this disastrous situation. He knew illusions were out of question, because the sharp eye of an auror would catch it in no time. He needed something better, but all spells that came to his mind were dark and those would give him away even faster than the illusions. As he ran near a dumpster, he decided that it was his best shot. He pointed his wand at the small trash bag and transfigured it into a rabbit. The animal quickly set off causing quite a noise and John in the meantime hid himself in one of the trash containers. He hoped that his magical signature which was now freshly imprinted on the rabbit would fool Potter.

He held his breath as he heard quickly approaching footsteps. He closed his eyes but the man ran past him. John kept listening, afraid that the man might return. But after a few minutes nothing happened. He let out a relieved breath and put his head in his hands. He was still too shaken to try to get on his legs and get out of the container. His mind and heart were racing.

To think that Potter would find him. John was so careful after the fall of the Dark Lord. The first thing he did was to run away to muggle London, hid his wand, change the money and pretend to be a muggle. He was good at playing the role. As a Death Eater who infiltrated St. Mungo's and fooled countless ministry officials, it was not a difficult task for him. It had been almost too easy to get a fake ID, telling some muggle officials that he has just returned from the war and got robbed of his on the airport. No one had asked question, no one had seemed to be interested in details. It was perfect. John knew that the best way to lie is to entail some truth into the made up stories. So instead of fighting a war against mudbloods and blood traitors, he said that he fought in Afghanistan against terrorists. If someone asked about his injures, then he would say that he got shot, hence his return to England. He doubted anyone would believe him anyway, if he admitted that he got hit with Reducto in the shoulder and that his limp isn't really psychosomatic but caused by too much exposure to Cruciatus Curse which Dark Lord simply adored to use.

After spending some weeks among muggles and making sure his cover was perfect, John even had gone as far as to attend a therapy. He had been bored. And not just bored. He had been bored out of his mind. Nothing had been interesting here, not even the Internet, telly or mobile phones which he reluctantly had learned how to use. He could not appear in Diagon Alley, because he was now a wanted man and frankly, he preferred to be bored for the rest of his life than to have his soul sucked by a Dementor, or worse – to be locked for years in Azkaban.

All changed when he had met Sherlock. He is going to be thankful for the rest of his life that he had run into Mike Stamford that day. They used to go to Hogwarts together and their families were on friendly terms. Mike hadn't picked a side in the war because right after graduation he started studying muggle medicine in order to improve various healing spells. That was how Mike got to know Sherlock at Bart's. And that was how he had decided that a strange, extremely brilliant but also extremely down to earth Sherlock Holmes would be a perfect flatmate for John. Someone who wouldn't believe in magic, who would be himself strange enough to actually focus the attention on himself instead of John.

But the extraordinary mind of Sherlock Holmes made John worry on various occasions. The Ex-Death Eater was startled when during their first meeting Sherlock almost completely guessed his cover story. John needed to be extra careful to not expose anything about the magical world he used to be a part of. The worry ceased during the Baskerville case when Sherlock made it clear that he doesn't believe in anything supernatural even if every evidence would point to that. This experience, though slightly traumatizing for John because of the drug induced fear of the hound, helped him relax and become more casual in his behavior.

So how come John is now sitting in a dumpster and trying to control his shaking body? The only answer he would give now would be: because he is stupid. Even more stupid that Sherlock believes him to be. And to think that he almost became a muggle. He dated muggle girls, even got married. Unfortunately, his wife had died during childbirth and his daughter hadn't survived the night. After that experience John decided to stop trying to get into a role of perfect muggle – married and with children. Instead he placed all his attention on Sherlock, with whom once again he decided to share a flat. Their life together wasn't easy, but surely it wasn't boring. The friends kept solving cases together and walking dangerous paths on daily basics. And that was perfect for John. It reminded him of his life as a Death Eater. Well, minus the murders of the muggles, mudbloods and other scum.

His life would be still perfectly normal if they hadn't got a case about serial killer who was murdering children in London. There were five bodies and Sherlock still couldn't find the motive or the killer. The thing that linked all the victims was that they died on their eleventh birthday and that the murderer carved a symbol of a snake and skull on their chests. John didn't need to think for long to connect the dots and figure it out who was behind the murders and what was the actual motive. The Dark Mark, hidden under think layers of Disillusionment Charms, tingled on his forearm as he first saw it engraved on the body of a fourth victim.

The case was extremely dangerous, but Sherlock wouldn't even hear about dropping it. So John had had no other choice but to retrieve his wand. Once he had done it, he couldn't help it but to use it. First, it were some minor spells – Alohomora, Lumos and so on. Of course, he was very careful to use magic only when nobody would notice it. But this night he and Sherlock were stalking a house where a boy would be celebrating his eleventh birthday. Devil wanted that Sherlock actually picked to watch a child who, as John had noticed, actually had some magical abilities.

John had prayed in his mind that this kid actually will be left alone by the murderer. But he hadn't been so lucky. He had noticed two men appariting in the back alley. He needed to act quickly, because Sherlock often behaved unexpectedly and this time it could end badly. First, he needed to get away from Sherlock.

"I need to pee. Will you be OK here by yourself for a moment?" All the response John got was a grumpy sound, but it was enough.

John hurried into an alley where neither Sherlock nor the two wizards would be able to see him. He pulled out his wand from his sleeve and pointed it at the child's house.

"Morsmorde!" he shouted as a green light burst from the tip of his wand and created Dark Mark above the house.

John used to cast this spell a lot in his past, and knew what reaction it would evoke. He hoped that the two wizards would flee from the place scared that now aurors would be arriving here. John only hoped that he would be able to run away to a safe place before that happens. He knew he wouldn't be so lucky the moment he heard a loud crack about fifty meters from the place he was standing. He also knew he was utterly fucked when he recognized the messy-haired auror with a bolt-shaped scar on his forehead. Without thinking he ran, and that was exactly how he got himself in the messy situation that required him hiding in a trash container.

Minutes passed and John finally convinced himself that Potter wouldn't come back. His legs stopped trembling, so he could at least stand up and get out from the damn dumpster. The man was too scared to use magic, thus he used his own strength to pull himself up and get out. His shoulder protested but the adrenaline which was still pumping through his veins lessened the pain.

As John stood next to the trash container he began to wonder what to do next. Going home wasn't an option because Sherlock was still around there somewhere. Surely he noticed the strange symbol in the sky and most likely was trying now to solve the mystery. Maybe police also was already at the almost-crime scene. And a handful of undercover aurors probably too. John's only option now was to find his flatmate, but before that he needed to come up with a lie that would actually fool Sherlock. A task almost impossible to accomplish.

"John!" His heart skipped a beat. Of course he was so lucky to actually be found by Sherlock.

"Sherlock, I…" He stopped in midsentence because he noticed that the genius detective was currently analyzing him. Saying anything at this point wouldn't help him. John braced himself for the blow, but tried to soothe his anxiety with the belief that his friend, while trying to understand the situation, wasn't taking magic or anything supernatural into consideration.

"What was that thing in the sky?" John tried to drag away Sherlock's attention. I must have worked because the detective blinked and then frowned.

"I don't know. I assume it was some kind of projection which was supposed to either scare the murderer or to notify the police. Maybe both. Or a job done by an accomplice to warn the murder which would mean that they spotted us." John stared at his friend with opened mouth.

"Amazing." He said. This time, however, John wasn't praising Sherlock's deduction skill but the ability to cast away without any problem anything supernatural.

Sighing, John let go of the rest of his worries and once again assumed the role of a muggle. He learned from Sherlock that shortly after the Mark appeared in the sky, the police came. Unfortunately, his friend wasn't allowed to investigate the case any further by some strange-looking, divorced policeman who, according to Sherlock, had serious problems with drinking. John assumed that it must have been an auror. Both of them were also told to come to a police station and testify. The eccentric detective hadn't agreed to talk to anyone excepted Lestrade, so at least John didn't have to worry about running into the aurors. He knew that they would recognize him immediately. In the Wizarding World his wanted posters were still decorating the busiest roads and bars.

John and Sherlock started to walk towards the main road. Only then the shorter man noticed that he smelled funny.

"I need to shower before we go to the station." He said.

Sherlock didn't respond, just kept observing the road. His eyes shone as he spotted a taxi. He waved to it and as both of them entered the car, he told the cabbie their home address. So he was listening after all.

The trip home was uneventful. Both men were deep in thought and didn't exchange even a word with each other. John was still thinking about what would have happened were he caught in that alley. And Sherlock was trying to solve the puzzle of tonight's events. From the frown that marked his forehead, he wasn't making much progress.

Two hours later, John and Sherlock entered Lestrade's office. Before the man could open his mouth to greet them, Sherlock started:

"Murderer is a member of a gang or a cult. The later seems more probable, though. It's also almost certain that he doesn't work alone. The projection we saw tonight above that house probably was made by one of his accomplices. It was a warning about the police or maybe about us if we were spotted." The consulting detective took a deep breath and continued talking with his monotonous tone that only barely betrayed that he was actually very interested in the case.

"The symbol appeared shortly after John had left and then he ran into the murderer or into his partner. He was then chased and hid himself in the trash container. The suspects must have run away, and cleverly I must add, because the whole area was surrounded." As Sherlock was speaking he was looking John in the eyes, as if trying to read something out of them. John congratulated himself for not flinching. As he turned his head, he noticed that Lestrade was looking at him with worried expression on his face.

"But why would the murderer chase John? Wouldn't it be easier for him to pretend to be a passer-by?" Detective Inspector turned his head to Sherlock.

"Panic makes people behave illogically." John said before Sherlock had any chance to open his mouth. Lestrade looked at him and nodded. The War for Blood Purity, as John used to call it, often made him witness illogical behavior of people who were scared or in panic. He even behaved illogically a few hours ago. He had taken a great risk with the rabbit and it was a real miracle that it worked.

"John, have you seen anything that would be of use?" Lestrade asked. The question startled John who was deep in thought. He couldn't focus, but he hardly could be blamed for it.

"No, sorry. Only that the man was wearing a long coat or maybe a robe…" Sherlock's eyes flashed.

"A robe?" He asked not expecting a confirmation. He furrowed his eyebrows and appeared to be thinking about something.

"It would fit your theory about the cult, Sherlock" Lestrade pointed out. Sherlock ignored him. The man sighed, being already used to that kind of treatment.

"You didn't see the weapon either?" The detective inspector asked John hopefully. The investigation was going really slowly and the man was hungry for any information that would let them make any progress.

"I'm sorry…" John started but Sherlock cut him in.

"What weapon? The autopsy clearly showed that all the victims died of natural causes. Beside the strange carvings, which had been done post mortem, there were no bruises, cuts or any other marks that would indicate the use of any weapon. I told you before that we are dealing with some kind of drug or poison." Sherlock's tone clearly suggested that the man believed to be dealing with an idiot.

Greg Lestrade knew better than to argue or get offended. While dealing with Mr Holmes one needed to have skin as thick as steel.

"Would you mind to make a full testimony? Donovan would…" The detective inspector started but Sherlock cut him off.

"Absolutely not!" After that declaration the man stormed out from the office. John nodded at Greg and quickly left too.

As both men were walking through the corridor towards the exit, John was looking around nervously, wondering if maybe some of the aurors actually came to the station.

"Something is not right about this case." Sherlock said pulling him out from his paranoid thoughts.

"What else is not right beside a sicko who murders innocent kids?" John asked. Sherlock clicked his tongue.

"That is actually the least important thing. I can't understand what would be the murderer's motive. Even if we are dealing with a cult, there is something that doesn't fit the picture. " John didn't have a comment to that statement.

"I need more data." John was certain that he didn't like what Sherlock was just implying. "We have to pick another kid and watch their house." Yep. John didn't like it at all. But he could have guessed that Sherlock would want, after tonight's events, to repeat the stakeout.

"John, hurry up! Let's go home and pick another house!" The prospect of hunting a murderer again visibly cheered Sherlock up, which made John smile unconsciously.

"But first let's get something to eat. All that running deprived us both of energy." John said in his doctor voice. Sherlock rolled his eyes and said:

"Dull." The shorter man knew better than to be offended.

"How about that Chinese restaurant that you like?" The offer seemed to please Sherlock because he said nothing. And as they exited the building he turned towards the path which led to the restaurant.

John was tense during the rest of the evening. He was observing his surroundings like a hawk for even the littlest signs of magic. The poor man almost got a heart attack when the Knight Bus passed them right in front of the restaurant. If Sherlock noticed that something was wrong with his friend, which he probably did, then he said nothing about it.

In the end, nothing extraordinary happened for the rest of the night. John hoped that Potter hadn't recognized him and just had thought that he was some dark wizard who cooperated with the murderers of the young mudbloods.

The peace returned to Baker Street. Well, if chasing criminals and doing weird experiments with Sherlock could be called peaceful. John decided to not use magic at all, at least for a few months. Lying low was the smartest course of action which seemed to work out just fine.