Work Text:
The first rule is never talk about it. There is business, and there is personal. The two don't ever coincide.
They meet on the battlefield. It's a war of intellect, of fierce smarts combined with foolish risk taking and dare, each of them trying to outwit the other without getting burned in the process. They pretend that it's a game of chess, as if they are two opponents trying to strategically outsmart the other with clever manoeuvers and underhanded tricks. In fact, it's more like they are building a castle of cards, or perhaps playing jenga, whereby at any given moment the entire thing will come crashing down around them. Sherlock and Jim aren't a couple of boring, stuffy old academics sitting in a dusty room moving chess pieces around a board and talking about the weather. God no. Rather, they are a pair of sharp minded maniacs having a laugh, merely on a grander scale than most.
They meet in private. Sometimes it'll be somewhere innocuous - a hotel room or in Jim's flat. Other times it'll be somewhere more daring - at the pool in the middle of the night, or perhaps in Sherlock's bedroom while John reads the paper right outside the door. During these times, they let the walls down. Forget consulting criminal psychopath, forget consulting detective sociopath. Their respective career paths are irrelevant, and they just let it be. 
They mix it up. Sometimes, when one or the other is feeling a tad vulnerable, they'll take it slow. Gentle caresses and soft, open mouthed kisses, fingers flexing in hair and slow, passionate movements. It's all about being wrapped up in one another, about feeling the closeness and the heat and the comfort. And so what if Jim is a little bit tender-hearted under it all? So what if Sherlock craves physical contact, despite spending his entire life pushing it away?
Everyone has their weaknesses.
Sometimes, when they're done, they'll lay there for hours and just stare at one another, silently thinking about what could have been, in another life.
Other times it'll be all about raw, violent passion. They'll collide with one another with ferocity, their kisses will be angry with teeth and tongue and a faint, copper-like taste. Fingers will grab and squeeze and claw, nails shredding through skin. They'll pin each other to walls, to the floor, to anything. They'll fight for dominance, break things and howl at one another. They'll be rough. The sex is hard and fast and, let's be honest, the pain is equalled only by the pleasure, and neither of them protests to it. They'll fall on their backs, panting, until one of them bites the bullet and gets out of bed, gets dressed and leaves.
It doesn't matter who's on top, Sherlock isn't in control, and neither is Jim. Their 'relationship', if you can call it that, is a wildfire: it'll destroy everything in its path until it burns itself out.
There are only a few rules. They don't talk about their respective 'live-ins' (as Jim affectionately calls them), they don't talk about Jim's scars or Sherlock's track marks. They don't talk about work in any great detail. God forbid either of them use the "L" word. It doesn't need saying.
Jim calls Sherlock his 'boyfriend'. Sherlock calls Jim 'his'.
Jim calls Sherlock 'sweetheart'. Sherlock calls Jim 'James'.
Sherlock is listed in Jim's phone as 'The Pretty Boy Detective'.
Jim is listed in Sherlock's phone as 'Gay Jim'.
In a way, it's just like any other relationship. Well, secret affair. John suspects but doesn't know. Sebastian knows but doesn't care. No one cares if Mycroft knows and the police just can't know.
They tease one another. They laugh about ordinary people and their funny little habits. They joke about criminals who are 'bad at their jobs'. They eat together, sleep together, shag or 'make love'. They share kisses and embraces and lazy days watching crap television, simply enjoying one another's company.
Like the calm before the storm.
It works. It makes them happy, keeps them distracted. When Jim is with Sherlock, he isn't scheming. He isn't plotting the downfall of governments, grand jewellery heists or assassinations. When Sherlock is with Jim, his mind doesn't claw itself to death with boredom; he doesn't crave cigarettes or feel tempted to slip into old drug habits. In a sick sort of way, they complete each other.
They know it's coming, though; The End. Jim calls it 'Reichenbach', Sherlock isn't quite sure why. He doesn't give it a name. He doesn't even want to think about it, despite knowing full well that it's inevitable. Their end is drawing closer and closer - it's going to happen whether he likes it or not. He often forgets that Jim isn't just Jim. He's Jim Moriarty. He's a dangerous criminal and Sherlock Holmes is the only man who can stop him.
Jim's criminal web is vast and complex. Lock him in a prison, he'll take over or break out. Shut him in an institution, and he'll refuse to swallow pills or vomit them back out when no one's looking. If there is a way out, which there always is, then Jim will find it. There's only one way to stop Jim Moriarty.
They say that death is only the beginning, but to Sherlock, Jim's inevitable death only serves to signify the beginning of the end.
~
They fight.
Jim comes to the house in the middle of the day, and it's by pure luck that Sherlock answers the door rather than John. He's been smacked around, there's blood in his hair and on his suit. He looks sad and defeated and hurt. All he really wants to do is reach out, lay down and wrap himself in Sherlock where it's safe and warm and clients don't want to kick his head in.
... But Sherlock yells.
He asks him what he thinks he's doing at the house in the middle of the day, who he thinks he is. He asks him if he realises what would have happened if John had answered the door instead of himself. Sherlock's always been terrified of John finding out about this, because he knows that he may never be forgiven for it.
Jim's taken aback and probably a bit concussed. He breaks the golden rule and insults John, tells Sherlock that his flatmate is an idiotic waste of space.
Sherlock retorts by calling Sebastian 'Jim's boy toy'. He looks at Jim in disgust and accuses him of being a little slut, even though he's certain the two aren't shagging because Jim would never do that to Sherlock.
Jim tells Sherlock that it's over, that he's tired of being Sherlock's 'substitute fuckbuddy' because 'John wouldn't ever touch him'. He tells Sherlock to 'go back to his junkie bullshit', because Jim's 'had enough'.
Sherlock completely loses his temper and brings up the scars that litter Jim's body, individually deducing everything he can about every single one, tearing them open with his words and shoving every nasty experience that Jim's ever had right back in his face.
Jim stands there, stunned, bleeding. There are tears in his eyes. It's the first time Sherlock's ever seen him look properly vunerable, and he immediately wishes he could take the words back, pull Jim into his flat and get him cleaned up, hold him in his arms and press kisses to the back of his neck. Pretend that they're normal; pretend they're alright.
Guilt. Regret.
He's never experienced either before, and he doesn't like the way he's feeling at all.
A taxi chooses that moment to pull up behind them. Jim turns and gets in it before Sherlock can stop him. The cabbie protests, tells him that he's been privately booked, but then Jim reaches into his pocket and passes him what looks like a small roll of twenties, ordering him to ' just drive'. He does, and Jim never looks back.
Sherlock doesn't hear from Jim after that. He sends him a text every now and then, but he never gets a reply. After a month or two, he gives in and tries to call, only to find that the number has been disconnected.
Months later, he's given a case. It regards a stolen painting featuring 'The Reichenbach Falls'.
Reichenbach.
It intrigues him and he takes the case.
It's probably just a coincidence.