A/N: Set the night before Christmas Day at the Holmes's - spoilers for series 3, episode 3.

When Mycroft opens his eyes, it is dark.

Pitch dark, actually. Unlike London, his parents home is in the middle of the countryside where there is no noise and, more importantly, no artificial light. Mycroft keeps two lamps on all night in his bedroom at home, but here he keeps on four. And yet all of them are off.

There is no light. That's why he woke. There is no light. There is nothing.

He has one coherent thought before the panic sets in, and that is it must be a blackout. And then the horror swamps him, thick and heavy and cloying and suddenly all his thoughts are deleted until only one remains, a constant litany of no no no nononononononononono –

He wants to run, but all his muscles have frozen, and anyway, where could he run to? The dark is everywhere, it seeps into Mycroft like water, steady and inescapable. It is coming into him and he is too afraid to fight it. The voice is screaming in his head, making it impossible to think. Nononononononono -

He's had this fear since he was very small, and he has forced himself to battle this demon, because men who are the British Government cannot have such an easily exploitable weakness. So far he has managed to train himself to sit in dimly lit rooms and feel fine, even safe. He spends as much time as he can in darkened rooms, watching blackness lurk at the edges of the light and forcing himself to stamp down the animal urge to run and keep running. But pitch black rooms, rooms without any light at all…those are another matter.

Nothing happened to him to make him fear the dark. God knows the world is dark enough in his head, he shouldn't be afraid of the darkness outside. But he is. Always has been. When the world is dark, it is empty. Mycroft is empty. He feels everything that makes him him slip away, trickle out of his head like sand. His great mind, so much greater than anyone elses in the world, falls to jelly, becomes formless. He can't focus.

He knows what being in the dark is like. Being in the dark is like being blind. It is like being a normal person.

There are stages to this. First, the blind panic, the inescapable screaming in his head. Then the freezing of all muscles, locking him into place. Now his breathing is quickening and roughening. He can hear it, a guttural, hoarse inhale and exhale, far too fast, as if he's dying. And then he wonders if he is dying, if he really is dying this time. Surely he can't keep up this screeching, harsh attempt at breathing for much longer without just…stopping.

The darkness is flooding into him. In a minute, one of two things will happen: either he will stay locked in position and start screaming, howling like a wounded animal on and on and on until the darkness stops, or he will curl up as tightly as he can with the little energy he has left and start sobbing in real despair. Both instances make him feel thoroughly ashamed when the lights are on and it is all over.

He feels his previously frozen legs twitch, his body curling around on its side, and realises dimly, in some far flung corner of his mind, that it will be the latter instance tonight. He presses his face into his pillow and waits for the hot tears to come.

Someone opens the bedroom door behind him. He can hear it, almost unnaturally loud in the dark, and the fear triples and quadruples inside him. He wants to scream but he can't even make a noise, though his breath is rattling around inside his lungs, squeezing his chest in half –

A warm arm curls around his waist, and he freezes in horror, a rabbit caught in a trap, and then he hears Sherlock's voice say, "Mycroft, it's me, it's me, it's okay."

Something uncurls in Mycroft's chest, enough for him to take a deep, heavy breath, the oxygen rushing to his head. Sherlock's arm is solid, overly tangible, around his waist, and his breath is tickling his ear. It's his little brother, Mycroft thinks, and is momentarily proud of himself for making that simplest of deductions.

"Listen to me," Sherlock says in his ear, softly and calmly. "It's just a blackout. The lights will come on in a minute. Everything will be fine."

Mycroft is still breathing hard, shuddering with the effort. Sherlock's arm tightens around him, not too tight.

"You're home, Mycroft," he continues soothingly. "You're perfectly safe. You're here with me. Remember home?"

Mycroft doesn't. He remembers nothing except the darkness. Sherlock may be here, but so is the dark, and it is pressing in on them both, crushing them –

"Myc," Sherlock says softly, in a little boy voice, dropping the word like clear water into Mycroft's ear, and then suddenly things are more focused. He remembers five year old Sherlock running around the living room in a whirl of black curls, getting in his parents way and interrupting Mycroft's reading. He remembers it as if it were yesterday.

Somewhere, he finds the energy to clutch at Sherlock's arm, twined safely around his waist, his fingers scrabbling at his brother's wrist. "Sherlock – " he manages hoarsely. He can feel the tears come to his eyes, and clutches tighter to his brother. "St – stay – "

Sherlock snorts, sounding normal again. "As if I could go anywhere with mother keeping John and I hostage here. Honestly, one tiny gun wound and suddenly it's family Christmas, my boy, and there's nothing you can do about it. If I'd known, I'd have made more of an effort not to get shot." There is a considered pause. "Or I would have just stayed dead."

Mycroft is only half listening to Sherlock's muttering. He clings to Sherlock's arm like a lifeline, one hot tear dribbling down his nose. He hears the door open again and a voice that should be familiar to him murmurs Sherlock's name. Sherlock shifts on the bed, awkwardly because he is keeping his arm tightly clamped around Mycroft, and takes something from the person. Something that emits light.

Hope gathers inside Mycroft. He raises his head, but Sherlock moves swiftly around and places the source of light on Mycroft's bedside table, right in front of Mycroft. It is a candle, a simple candle in a candle-holder. The flame is so bright.

All the breath in Mycroft's body escapes him in one long breath. Sherlock murmurs something to the person at the door, who vanishes again, closing the door behind them, and then he tuts right by Mycroft's ear and says, "Remember to inhale as well, Mycroft."

Mycroft obeys, keeping his eyes on the candle flame. It is the most beautiful thing he has ever seen. He inhales and exhales again.

"Good," says Sherlock, sounding more cheerful. His arm does not leave Mycroft's waist, even when Mycroft is breathing more or less regularly again. Mycroft suddenly feels very tired, and his face is wet. He glares fiercely at the candle, watching the darkness being kept at bay by that small orange light. He can keep it at bay as well, he thinks. He will, one day.

Sherlock nuzzles his noses against the back of Mycroft's head, probably more because he wants to have Mycroft's attention again rather than to calm him. Mycroft speaks, and his voice is reedy and thin, but at least he can say something.

"You came to help me," he says, weakly.

He feels Sherlock hesitate as he struggles between sentiment and sarcasm. Eventually, he just says, "Mmm."

It is a bit of a daft thing to say. Sherlock has always helped Mycroft at times like this. Ever since the first time, when Sherlock was three and Mycroft was ten and the lights had gone out in the house. Ever since little Sherlock had climbed onto Mycroft's lap while he hyperventilated and had said "Mycwoff, no" and seized hold of his hand with tiny, sticky fingers. If Sherlock was around and there was darkness, he would go and look after Mycroft. Even if he wasn't around, actually. Once there had been a blackout in London while Sherlock was in Edinburgh, and Sherlock had called up 'Anthea', demanded she buy at least five gas lamps and deliver them post haste, then had rung up Mycroft and talked to him for seven hours straight.

Sherlock is not one for sympathy, but he can, unexpectedly, be empathetic. Mycroft has always suspected that Sherlock harbours a little bit of the fear as well. Or he at least knows what Mycroft feels, knows of that blank emptiness that takes over Mycroft's mind when the darkness falls, knows of that feeling of normality. How can normal people be like that? So blind and uncomprehending of what is in front of them? Feeling their way through the darkness for the entirety of their lives, and not knowing it because it seems so normal to them? It is horrifying.

Mycroft focuses fiercely on the little flame. The dark is still there, and the fear, but he has enough light in his mind to control the panic now. He trails shaking fingers across his brother's wrist, wipes his wet cheeks on his pillow.

"Thanks, Sherlock," he says. It comes out so honestly, not dripping with sarcasm or riddled with other motives like most of his words these days. That was ten year old Mycroft speaking there, showing simple gratitude to his tiny, curly-haired annoyance of a brother who had helped him even when he hadn't understood what was going on. He wishes he could be more like ten year old Mycroft sometimes.

"Mrrrrmph," Sherlock says drowsily in reply. His head is lolling in the nook of Mycroft's shoulder and neck; he is falling asleep. Mycroft doesn't quite have the confidence to reach a hand out of the bed and check his phone, but he suspects it is about four in the morning or something. It is Christmas Day, he realises.

"Merry Christmas," he says to Sherlock. Sherlock, in reply, snores a little, already asleep.

Mycroft traces patterns around his brother's wrist in silence. In a minute the lights will come on properly and he will feel a rush of annoyance and humiliation that his age old fear got the better of him yet again, and this irritability will carry on through tomorrow, making him grumpy and uncommunicative. He wants to be at home in London, not here where it can get so dark without any notice. He hates being out of his comfort zone. And he detests Christmas.

He wishes he could thank Sherlock properly, but he'll probably spend the whole day bickering with him instead. He'll say 'Merry Christmas' to him though, he decides firmly. He'll at least say that.

With a flicker, all the lights come back on, and the room is once more awash with a warm, golden glow. Mycroft feels himself relax properly, then feels his frustration with himself grow. The world is once more real again and he feels like a complete fool for letting his anxiety get the better of him in the first place. It is like turning on the lights after watching a scary film – everything is suddenly normal and in place, after everything was so disjointed and frightening. It's a sort of anti-climax.

Mycroft glances at the candle, wavering between being safe and blowing it out and worry that the lights might go out again, but he decides that if the darkness does descend, at least Sherlock will be right there, and the dawn can't be far off now. He blows the candle out and snuggles down, drawing Sherlock's arm more securely around his waist. Sherlock is far away, murmuring softly under his breath, breathing through deductions even as he sleeps. He has always been like that.

Mycroft looks around one more time, just to be sure, and then he closes his eyes and sleeps.