It happens, quite literally, overnight. It's that fast. The flicking of a switch that he hadn't even known was there.
It's also, at least partially, accidental.
It had been an unusually busy day at the Institute. Alec had had to re-organize patrols after signs that long-dormant demon hot-spots were becoming more active, induct new recruits visiting from the Shanghai Institute, investigate declarations of missing inventory from the weapons cache, check endless reports, and make his own to the Clave cohort in Idris. He hadn't stopped once – and to top it off, Magnus was travelling for work, and wouldn't return for at least two days. Although Alec had a key, the loft wasn't the same without Magnus, and Alec didn't relish the thought of an empty bed.
So it was that, somewhere in the small hours of the morning, Alec finally, reluctantly, dropped onto his bed in the Institute dorms – and that was when it happened.
His head swam. His ears rang. He was overcome by a full-body rush of dizziness and fatigue, and though the world was lost to darkness, he couldn't tell if his eyes were open or closed. For an instant, he was weightless. He felt completely untethered. Light.
Free.
And then, he crashed back into his too-solid body, into aching muscles, a pounding in his temples, and a cramping in his stomach.
Oh. That explained it. He hadn't eaten today. He genuinely hadn't had time, had forgotten to think about anything but the work in front of him. No wonder he'd gotten so much done. No wonder he was so blissfully, drugged-ly, dazed right now. Neither of these were unpleasant things. There was comfort, and something like pride, in a productive day, in this hard-won fatigue. It was solid proof that he'd achieved something.
Knowing that he should at least make sure he drinks some water (he doesn't), that he should change his clothes (herculean, impossible - unnecessary), and wondering if he can repeat this performance tomorrow (he will), Alec just remembers to kick his boots off before falling into a dreamless sleep.
Click. The switch.
It's that fast.

The next day goes surprisingly well. Despite Magnus's efforts ('breakfast is the most important meal of the day', 'many a mission to end a great evil has failed because of low blood sugar'), Alec has never been one for big breakfasts, preferring to sleep in when he can, and to get a head start on his day when he can't. He dislikes the sluggish feeling of training after eating too heavily, and he's never felt as though he was depriving himself by devoting minimal time to his diet. Magnus has discerning tastes, an appreciation for the rare and the high-quality as well as the nostalgic. Izzy is the one who gets excited about cooking, experimenting with new recipes and gathering everyone over an unpredictable assortment of dishes whenever the opportunity presents itself. Alec's attitude to food has always been indifferent. Food is fuel. He has his favorites, sure, his go-to choices, but even these preferences aren't very strong. He enjoys indulging Magnus and Izzy's efforts to refine his palette, but mostly he just doesn't get the fuss.
It's easy enough, today as on most mornings, to stick to coffee and throw himself straight into work. Perhaps because of this, the hours seem to fly. He's just as functional, if not more so. He feels sharp, focused. There's nothing to distract him from his job.
If he doesn't need breakfast, it follows that he can skip lunch, and dinner. The whole idea of a sit-down meal, or a full plate of food, starts to strike him as not only inefficient but wasteful, greedy. He's never brought into the outdated idea that there's anything special about his angelic heritage, but if he had even a fraction of that strength, that grace and energy, he wouldn't need at all. It's his mortal half that's weaker, that needs maintenance; but maybe he can train that part of him to endure, too.
Besides, by the afternoon, there's a pleasant, clean emptiness inside him that makes everything else easier to bear. He likes it. He wants to protect it. And when he falls into bed at the end of the day and his body starts to clamor and protest every gnawing, cramping pain, he likes that, too.

From then on, it's simply something Alec does. Or rather, doesn't do.
He doesn't eat.
It's easy, as long as he stays busy, and keeps to himself. Occasionally, Jace might throw him the last part of an energy bar, or Izzy will force him to taste-test her latest creation, and when he minds Madzie for Cat and she wants to experiment with Mundane baking, he's not so heartless that he refuses to choke down a cookie that's more icing and sprinkles than biscuit – but, on the whole, he does well. He never eats alone, and never more than a bite or two in a sitting. Magnus is delayed on his trip, which is several distinct kinds of awful, but something of a blessing, too. It's one less person to pretend for.
Well, not pretend for, exactly. Alec isn't faking anything. Not even going out of his way to hide his new routine. But he knows that this is something that he doesn't want to draw attention to. It makes him feel safer, somehow, to have this one part of his life he can control, keep private, and there's a strange kind of pride that comes with keeping a secret, testing himself.
And he isn't hurting anyone. Not even himself, not badly; he feels okay. Yes, he's almost constantly tired, and his body aches, everywhere. Yes, there's an insistent pain in his head that doesn't ease even after trialing Mundane painkillers. Yes, he's learnt to pause when he stands, because the mini blackout brought on by sudden movement is just something that happens now. And alright, a lot of the time the world retreats behind a screen and he struggles to read, to write, to shoot -
But the rest of the time he's untouchable. The world is sharp and clear, stripped of everything surplus. And he doesn't even mind the foggy moments, really, because they blunt his anxiety. He compensates by working harder, drilling himself in combat techniques until everything is muscle memory, and it feels right, to take ownership of this skin he's never quite felt comfortable in. To discipline his body into sparse utility and make the space he occupies worth… something. He's spent so long following other people's rules, living for everyone else, and whilst he doesn't begrudge this, it seems only fair that he gets to set some boundaries of his own.
And nobody says a word. He doesn't think they notice. It isn't until Magnus has been away just over a week that the first alarm sounds.
He's sparring with Jace one afternoon, and Jace is notably slower, putting less power behind his hits. Clary, watching, teases him, but he snaps at her, and remains grouchy for the rest of the session.
'You okay?' Alec asks as they clear the room for the next users.
'Yeah, I guess I'm just tired.'
'Anything on your mind?' Alec asks.
'No.'
'Yes.' Cary interjects. 'And I know what it is. See that face?' she says to Jace, raising her hands and making a frame of her fingers. She squints through it as if focusing a camera lens, and Jace thaws ever-so-slightly at her playfulness. 'You're hangry. Even after we pigged out at brunch with Luke.'
'You got Jace to brunch?' Alec asks, snorting at the mental image of his brother glaring suspiciously at a bellini – but it vanishes, quickly replaced. Alec thinks: juice. Pastries. Eggs. Cereal. Fruityoghurtbaconbread –
Courses. Buffets. Bottomless.
He recoils, and when he scrubs a hand over his face as if to wipe away the images, his palms are clammy. In this phantom sensory onslaught, he is disgusted by his own greed, by the idea of indulgence.
Not necessary, he tells himself, and focuses on his parabatai.
Jace scowls.
'Carbs. Caffeine. I stand by it.' He states mulishly, but soon brightens as a thought strikes him. 'You're right, Clary. We should eat. Can't keep all this up without fuel.' He says, gesturing towards his currently bare torso and grinning.
'Whatever you say. Fuel all you like, I'll still kick your ass next time, too.' Alec taunts, though there's no malice behind the words, just familial banter as automatic as what he says next. 'You two go ahead. I'll finish cleaning up here.' He encourages, as if it's the simplest thing.
'You sure?'
'Yeah, I got this covered.' Alec says, and Jace frowns, studying Alec with an unreadable expression before Clary leads him away.
And they do go, Jace muttering a few words of which Alec only catches weird though and literally starving, because for them it is that simple. Their bodies need to recover. Re-fuel. They've worked hard, worked up an appetite, used stores of energy that need replenishing – and Alec gets it, he does. That's how it works. Indisputable, biological facts, even by their slightly altered Nephilim standards. No-one can run on empty. Of course, they need to eat, to take care of their bodies. If nothing else, it keeps them mission-ready. No-one sensible would question that. Alec knows this.
It just… doesn't apply to him. It can't. He needs to be beyond that.
His stomach clenches, and his hands shake as he clears and sanitizes the training room. He makes a mental note to mute the parabatai bond when he can, at least around Jace. He'd forgotten how his parabatai might be affected, how closely they're linked, and Jace had gotten too close to – well, just too close.
By the time he's done, his heart is still pounding. Alec tells himself he's just recovering from the workout, that he probably only needs to stop for a minute, but something else tells him that that can't have been enough. There must be more he can do. If it wasn't for the arrival of a trio of younger Shadowhunter's, clearly waiting to use the space but too nervous to approach him or nudge him out, he'd probably have stayed and run through some solo drills. Instead, he gives the group a nod and walks away.
Restless and unsettled, he escapes the Institute under the pretext of a routine reconnaissance trip, and walks aimlessly through the city until long after nightfall. The cold scrapes at every hollow and plane of his body, scours his lungs, and he imagines himself being carved away from the inside. Erased. The wind shattering, scattering, whatever shell is left behind.
He imagines dissolving, every mistake vanishing with him. It calms him enough to let him sleep when he returns.

The next evening, Izzy drops by his office carrying two plates and announces they've ordered food.
'You weren't around when everyone was deciding, but Clary wanted Italian and I figured pasta was safe enough, so I just chose for you.' She says, smiling. She's pleased, with that half hopeful, half proud expression she wears whenever she has a chance to look after him for a change. It gives Alec a pang, takes him back to some of his better childhood memories. His fiercely loyal, fierce little sister.
'Thanks, Iz, I just need to finish –'
'I know you're busy, hermano, that's why I came to you. Figured I could hang out here and keep you company.'
'You mean distract me.'
'Maybe.'
'I don't think we're supposed to eat in here.' He tries, and it sounds weak even in his own ears. Izzy scoffs.
'Your office, right? Your rules. Here,' she says, setting the plates on his desk and stealing a stack of the expense reports he's been reading. 'I can help.'
'Izzy –' Alec starts, but she's already making herself at home, settling in the chair across from him, dividing the work between them and nudging his plate towards him. She pointedly ignores him, already checking the math on the first report in front of her, and primly spearing a forkful of penne from her plate. 'Fine. Thank you.' He says, resigned. His office, yes, but apparently not his rules.
Alec goes back to the report he'd been reading, and there's a few moments of companionable silence. The smell of tomato and basil, always a homely, inoffensive thing, now seems vaguely malicious. He has to stop himself staring at Izzy, counting each bite she takes. He looks at his own plate. It seems huge, and unnerving. He's suddenly unsure of ingredients he's seen a thousand times, but he can imagine all too clearly the heaviness of each bite sticking in his stomach. The effort of chewing. The pure, echoing space in his center, erased. It's far from appetizing. He picks up his fork, stabs at some pasta. Scrapes it off again. Stirs the sauce. Levels it. Puts the fork down. Corrects a calculation on the document in front of him. Picks the fork up.
Puts it down again.
He physically cannot do it. Any of it. Bite, chew, swallow. There is no world in which he tastes this food, let alone clears the plate in front of him. He can't.
That's not a bad thing, he thinks. He doesn't want to, and he doesn't need to. So he won't. No-one can make him. Simple. He's an adult. His little sister shouldn't have to look after him.
She also shouldn't have to worry about him.
Your rules, Izzy had said.
Yes. His rules. His. Perhaps he was still figuring them out, but he knew that the heaping plate in front of him was a challenge, daring him to flout them.
By the angel, this is pathetic, Alec tells himself. Two competing voices.
Eat.
Why?
It's just food. Don't be childish.
You don't want this.
How will Izzy feel?
How will you feel?
Pathetic indeed.
'Alec.' Izzy snaps.
'What?'
'What are you doing? You were the one who always told me not to play with my food, and you're looking at your dinner like it's going to eat you. I already told you I didn't make it, so you know it's not toxic. Eat.' She grins, but there's concern in her eyes.
'Sorry.' He shrugs, shifting the plate to the side and picking up the next set of figures. 'Just not that hungry right now.' Surprisingly, this is true. The inexplicably nauseating smell, mentally revising the mechanics of eating… It's all left him uncomfortable and heavy.
'Well, okay. I'll leave it in the kitchen for you, but if Max or Jace get to it first, that's your loss.'
'I'll keep that in mind.' Alec smiles, and they work while Izzy eats, keeping up an easy conversation. Izzy doesn't say anything else about his food, but as she stands to leave a little later, she looks at the plates in her hand, one almost empty, the other untouched since Alec's initial attempts. She looks at Alec.
'When does Magnus get back?' she asks casually. Too casually.
'Friday.' Alec says through a sudden lump in his throat. 'Two days.'
'Not long.'
'Mm. Thanks for your help, Iz.'
'Don't worry, I know. I'm the best. Don't work too late, okay?'
'Never do.' Alec says innocently, and Isabelle rolls her eyes as she goes. Alec feels an instant, guilty relief at being alone. He knows he has more to do, but he finds himself staring blankly at the half-open door, unsure what to make of tonight. When he tries to go back to the paperwork in front of him, he finds he's creased a sheet in a clenched fist. He flexes his hand and smooths the damaged corner, scowling when it proves impossible to straighten completely. It irritates him, this imperfection, this irrefutable evidence of something no longer pristine. He stretches his hand again, trying to relieve some of the tension, and watches the play of bone and sinew beneath his pale skin. There's a slightly blue cast to the beds of his nails.
He sighs, knowing there's no way he can concentrate now, and decides to spend an hour or so shooting. He needs a distraction. The whole evening has left him feeling acutely unsettled, and he doesn't want to think about it, to wonder if he's doing the right thing.
Later, with bloodied fingers and a yawning pit inside him, the world blurring in his periphery, he decides that he is.

Friday morning is painful. Magnus calls to say he's finally home, getting settled back at the loft, but Alec has a couple of last-minute meetings he can't get out of, so it's almost noon before he can make his escape. The journey to Brooklyn has never felt longer, and by the time he's climbing the stairs to Magnus's door, he's burning with impatience and want.
It's been nearly three weeks. They haven't spent this long apart since they met.
He doesn't have to knock. Magnus is there, waiting, leaning in the doorway with a Cheshire grin.
'Hi gorgeous.' He says, coy and fey and here. Seeing him is everything, and Alec is a drowning man finally surfacing, dizzy with the influx of oxygen, of life. Without stopping to think, he closes the distance between them and cups Magnus's face, pulling him into a hard, starved kiss. Magnus makes a noise of pleased surprise and responds just as urgently, pulling Alec by the loops of his jeans through the door and into the apartment. They stumble a little and Alec smiles stupidly against Magnus's lips. His sudden joy, the rush of relief – and yes, perhaps the kissing – have made him lightheaded. He can feel his pulse, the too-quick, too-light stutter of his heart, everywhere. He pulls back just a little, trailing softer kisses from the corner of Magnus's mouth, along his jaw, his neck, finally wrapping him in a tight embrace.
'Welcome home.' He murmurs, voice a muffled somewhat against Magnus's shoulder. Magnus laughs and holds him back just as tightly.
'Careful,' Magnus laughs, 'anyone would think you missed me.'
'You have no idea.'
'I may have some.' Magnus counters, releasing him. 'Believe me, I missed you too. I'm sorry I was gone so long.' He adds softly, one hand reaching to trace the length of Alec's cheekbone. Something in Alec fractures at the touch, the warmth, the aching tenderness of the gesture. He exhales.
'You're here now.'
'So I am, and we have some considerable catching up to do, I'm sure. Coffee?'
'Sounds great.' Alec smiles. 'Tell me about your trip.'
'Oh. Well, I'm in no hurry to repeat it.' Magnus says with a sigh, and as he leads them to the kitchen and brews coffee by hand (one of the small Mundane rituals he enjoys), he tells Alec briefly about the legal dispute he'd been mediating between two competing High Warlocks, each of whom made claim to territory thought to be rich in certain rare potion ingredients. Local Downworlder's were beginning to get caught in the squabbling, some hurt, and it was only by translating some extremely old historical records and searching the territory himself that Magnus was able to resolve the case by establishing that the land had originally belonged to a vampire. Although the vampire had been killed (for the final time) sometime in the 1750's, his extended human family had thrived, and he, surprisingly, had a living, if distant, descendent. She was Mundane, but the land, it's properties and resources, were hers. Magnus had been delayed because he wanted to establish a mutually beneficial working relationship between the feuding parties and set up wards to protect the newcomer between them. It struck Alec as a bleak story, with no real victors, and he could tell it didn't sit well with Magnus either.
'Things don't really change, do they?' Alec said, frowning at the steaming mug in his hands. They sat across from each other, on stools at opposite sides of a counter, and Magnus was watching him thoughtfully, elbows on the countertop and fingers laced beneath his chin.
'Perhaps not as quickly as, or in the ways, we hope, but… we have to try, and do what we can in the meantime.'
'Yeah, of course,' Alec said. It was what he'd always believed (or wanted to), too. That if you just worked hard enough, if you kept trying, you could achieve something. Now, the idea of trying overwhelmed him. The world, their world, and all its conflicts and dangers and needs, was so vast. So heavy, and complicated. He'd heard people say it was best to focus on things that were in your control, but what was that, really? In his job, he did not have sole jurisdiction over his decisions and actions. He was a representative of his family and wider communities he was still learning about. A leader. Even his identity and his relationship were seen by some as a statement, symbols for them to interpret long before he ever came to any conclusions himself. There was no way to shed it all, to exceed or be free of expectations. 'Do what we can.' He echoed.
And what he could do was better. He could work harder, need less, take charge of the parts of him that remained his.
'Yes. But enough of my shoptalk. I didn't come home to cast all this gloom. What's new with you?'
'New? Not a lot, really.' He admitted. It was dull but honest. He hadn't had much of a life outside of his new… rules? Routine? None of it was conducive to socializing, or breaking the cycle of work, (try to) sleep, repeat. Now that he stopped to think, he found it hard to remember exactly how he'd filled his time while Magnus was gone, to separate one day from another. 'Work's been keeping me busy. We've had some new recruits, and they all seem to be settling okay. Mostly I've been putting out fires and trying to anticipate the next one before it starts.'
'An occupational hazard, pillar that you are.' Magnus says teasingly, but his gaze is studious in a way that puts Alec's guard up. It's another example of his unnerving ability to seemingly read Alec's mind at the strangest times. Magnus insists it's simply his age, and time spent with a list famous philosophers and psychologists that Alec is sure must be at least partly fabricated, but it's more than that. Alec has allowed Magnus to see him more closely, more honestly, than almost anyone else. He's been vulnerable.
Weak. Needy. Rein it in. Nobody wants this.
'Hah. Because the demons aren't enough.' he says flatly, unable to match Magnus's light tone. Pillar is right, in a sense. He can feel the weight of his responsibilities, his duties, everything he's tried to live up to, like stone after stone on top of him. Magnus doesn't reply straight away, just tilts his head, and folds his hands around his mug, painted nails (indigo today, one of Alec's favorites: it turns Magnus, blue and gold and silver, into a starry night) tapping against the ceramic.
'Fires.' He repeats eventually, voice even. 'Is that why you look a little burnt out?'
'Burnt out? Should I be offended?' Alec quips half-heartedly. Magnus rolls his eyes.
'Please. You know you're a knockout - but you do seem tired. Distracted. I can tell when you're working too hard. How have you been, honestly?'
Alec takes a slow breath, studying his own half-empty mug. The dark sheen of coffee, the faint glaze of the ceramic, show distorted fragments of his reflection, too faint to compare properly. He can't decide which is the truest. Which is him? Where is his face, and where is he, beneath it? He decides to attribute his racing pulse, the restless energy begging him to move, get out, go, to the caffeine. He smiles at Magnus.
'Fine, I promise. Just a few long days. I'll recharge. And… I really did miss you.' He adds, because it's the truth, even if it isn't all of it. He doesn't understand enough to give a fuller answer.
'I'm here now.' Magnus promises, reaching for Alec's hand. He strokes his thumb along the scarred knuckles. 'You're sure there's nothing else?'
'Yes. Yeah, I'm sure.' Alec says. He takes another sip of his drink, keeping his attention on the way the warmth spreads from his throat to his chest, pooling in his abdomen and radiating slowly. He feels both too far out of and too far inside himself, emotionally detached but unpleasantly aware of every physical sensation, the boundless weight and demands of his body.
He wonders what Magnus sees when he looks at him, how his shape, the negative space around him, translates to others. He meets Magnus's eyes but can't make out his own reflection in their shine.
'Okay,' Magnus says, 'but if you need to talk…'
'I know.' Alec replies, and smiles. To his relief, Magnus lets it drop for now.
He's fine. He's sure.
He's going to do better.
He has control.