This is a lovely little oneshot that sprang out of nowhere while discussing theories with Latest From the Asylum and DoctorTheTwitch. I posted it here because I enjoy ripping your little hearts apart more that I probably should.

Pyxis

It had been one hundred years and a month and a week and a day since Magnus had done the impossible, the thing that he'd been afraid to do ever since he'd heard it could be done. One hundred years, and now the time was up. He could feel it, feel himself getting weaker, and he didn't care. He didn't care at all. He'd known for one hundred years that this was coming.

Alec, long ago Alec, nearly but not quite forgotten Alec, had persuaded him to do it. Together, they had built a Pyxis, runed and strong and big enough to hold a Greater Demon. Together, and not easily, they had used spells and potions to coax the hated demon essence from Magnus's body. The Pyxis, once it had the demon essence trapped inside it, ticked away the minutes like a clock. One hundred years, Magnus said, listening to the box. One hundred years and a month and a week and a day, that's how long it will last. That's how long I have. And he'd smiled, because he knew what it meant.

Alec didn't realize it, not really, because to a nineteen-year-old one hundred years is forever. All that it meant to him was that his beloved Magnus was finally human, had gotten rid of the piece of himself that he hated above all else.

But Magnus knew better. And with the help of that knowledge, he got up the courage to ask Alec the question he'd never dared to ask anyone before. And when Alec, darling wonderful beloved Alec, said yes, it helped him slip first an engagement ring and then a wedding band around his finger. It helped him through his life with Alec, the life that he'd never thought he would get the chance to live. When Alec was pigheadedly stubborn and refused to admit that he was wrong, it helped him remember to be patient with his husband. When he got so sick once that Alec had to carry him to the bathroom and back, it helped him live through the humiliation.

And when, six years later, when Alec was injured in a demon hunt, the knowledge helped him through his lover's death. It numbed the pain somewhat, though it was still an ice-cold knife to his heart to see his husband lying stiff and still in the coffin. His Alec, still young and beautiful, without the chance to age.

Alec, whom he'd loved with all his heart.

And Alec, the last person he would ever give his heart to.

It helped him wait through nearly a century with stoicism, because he knew what lay at the end of them. After a decade, though he could still feel the ache of his death, Alec's face became shrouded in memory. The years blurred his features, and Magnus often found himself wishing that he could recall the shape of Alec's nose, the curve of his neck. Sometimes, at night, he dreamed of eyes blue as the Caribbean Sea, watching him from across the room. And sometimes, when he woke up, for a moment or two he could not recall to whom those blue eyes belonged.

Then even the dreams faded. Once, drunk and half-asleep, he began to wonder if Alec had ever really existed. He wondered if maybe it was a trick his brain had played on him, make-believing a boy like the lost and forgotten Will for him to love. He reprimanded himself for these thoughts the next morning, but as the years rolled on he found himself having to visit Alec's grave to remind himself that he had been real.

At ninety-seven years, he began to wonder if his prediction had been correct. The time was nearly up then, and there were still no signs of the end. The Pyxis, which he had carried around with him since Alec's death, ticked steadily on.

At last, just when he'd begun to think it hadn't worked, he felt it. A tremor in his hands, at first. Then a stuttering in his chest. He began to have trouble breathing, and to wake up in the morning feeling weak at the knees. Over the next several months, his hair turned a soft silver. Lines began to appear in the soft skin of his face, though he still had the body of an eighteen-year-old. And every time he noticed a new symptom, he smiled to himself because he knew he was one step closer.

The ticking beat that he lived by began to stutter, first only once or twice in a week and a few times a day and then every hour. One morning he woke to a half-hearted tock every half hour or so.

That morning he knew it was the end.

He did the things he knew must be done—found places for his pets and his belongings, made arrangements to be sure that his magical supplies fell into the right hands. One hundred years and a month and a week and a day, exactly, and he knew it was time to go.

And suddenly he found that he was very reluctant. He knew there was not much time, and yet instead of Portaling he booked a flight, first class, on the first plane that would take him from Los Angeles to New York.

He packed with care, and yet he took no clothes but his finest silk shirt and dress pants and a tie. Instead he filled the suitcase with mementos from his short time with Alec, the happiest time of his life. He took his time with each, letting the memories that they brought wash over him.

A receipt from Taki's, reminiscent of their first date.

Magnus's blue scarf, the only thing of color that Alec would wear and that brought out the blue of his eyes.

Alec's engagement ring, that he had warned on their wedding day Magnus better take care of, or else.

The newspaper announcement of their wedding that Magnus had insisted on.

All six of the cards that Alec had given him on their anniversary.

And last, a pressed white flower from the bouquet that had lain on Alec's chest before his funeral.

In New York he walked the paths that they had so often walked together, mentally pointing out the places that they had been. He went last to the old flat, now so tumbled down that the city had forbidden it be lived in. Magnus, who never cried, felt tears pricking the back of his eyes as he looked at the building.

He couldn't stand it anymore, and anyway he didn't know if he had the strength for the flight to Italy and then to Idris. So he drew a Portal in the air, draining the last of his magical strength, straight to the graveyard.

He staggered out of the other side of the Portal, gasping. He couldn't catch his breath as he stumbled through the gravestones, searching for the one marked Lightwood. His legs wouldn't hold him anymore, and he collapsed as soon as he found it. With shaking hands, he laid the mementos he'd brought against the stone.

His vision was graying as he lay down beside the grave, crossing his hands over his heart. The Pyxis beside him gave a tick and a scraping noise. It occurred to him what a scandal this would be, a warlock dying in the Shadowhunter graveyard. The thought amused him.

He closed his eyes; it was too much work keeping them open. The silence was so deep he could taste it. "I'm coming for you, Alec," he whispered. "After all this time, I'm coming for you."

Beside him, the Pyxis gave the creaking ping of breaking machinery.

With his last breath, Magnus smiled.