Notes: /Title taken from Neptune by Sleeping At Last, which doubles as a theme/

So! This was meant as a one-shot, but I decided to split this in half so I don't mix Jace and Alec's POV in the same chapter. Pretty much what it says on the tin; I tried to sneak in a little worldbuilding here and there. I'm actually proud of this one so far, which is... an achievement given my current schedule, so I hope you guys enjoy it and feedback is always welcome!

Jace learnt how he was going to die the day he learnt how to read.

It had been a tough, lengthy process; not just memorising which letter represented which sound, but putting them into words and sentences too, and it had been worth it. Not just because it pleased his father, now, but because that meant that he could finally understand what his soulmarks meant.

They were simple little scrawls, intended for his eyes only. He'd read all about them afterwards and knew what they represented the last words his soulmates would ever say to him. They all seemed rather laconic - typical for Nephilim even in the face of separation and death, his father assured him - and Jace had just started imagining what they would be like when he got to the part of the textbook that explained why one of his marks was so much paler than the other two.

I'm sorry, the faded proclamation on his left forearm said and whoever had spoken those words was long dead.

It had to have been his mother. His father rarely ever mentioned her and in a way, Jace was glad for this scar - it connected him to her in a way nothing else would. It was a muted, distant sort of grief that overcame him when he looked at it, but apparently that was to be expected too. It was just the complicated nature of what soulmates were and what they meant; the realisation was always inevitably tied with loss.

And still, Jace knew that he would meet his other, living soulmates; would be loved by them, even. He'd never talked about his soulmarks to his father and had insisted that it didn't matter because it was all Michael ever repeated, but he knew that he wasn't alone. Just above his right elbow, the words Be careful were etched into his skin and were accompanied by Just look at me a little higher up. They brought a strange amount of excitement with themselves - he would die in battle, Jace knew, and to someone out there - still foreign and undefined - his death would matter enough for them to worry. It would be painful in that case, of course, but that was all love was about anyway, wasn't it? It would be perfect.

o.O.o

Be careful, Jace quickly discovered, was one of Alexander Lightwood's favourite things to say. Once they'd grown closer, he said it all the time - in alarm, when a demon got too close; angrily, when he thought Jace was being reckless; fondly, even, as he cleaned blood and ichor from Jace's face. He said it so often that in the end, Jace almost stopped being startled by it.

It made him love Alec a little more every time it happened, but he'd managed to convince himself that it didn't matter - these weren't parting words when they came from him, it was just Alec being Alec and there was no way that those would be the last words he would say in his presence. Jace's idea of what the future would be like wasn't too clear, but he would not die on him where Alec could see; he'd decided that much. It was the one thing he strove for, because while he was ready to concede that his father was right and that it all always ended in heartbreak, he would never break Alec's heart if he could help it.

Despite that, it was distressing to realise that Alec agreed with his father too, albeit unknowingly. The Lightwoods were a private bunch with the exception of Izzy and they kept the natural glamour over their soulmarks active at all times. Isabelle had been the only one to go through the effort of showing her own to him - she had four - but Alec had refused point blank.

"It's completely meaningless either way." He looked irritated and not the least bit excited by the prospect. "Why does it matter? I won't know for sure until they're gone and by then, my marks won't matter anymore because they'll be gone."

He had more than one, then, and all were alive. Jace breathed a sigh of relief. He could see where Alec was coming from - since he'd arrived in New York, he'd heard stories of all sorts of marks for all sorts of relationships, sometimes lifelong, sometimes as ephemeral as a mundane saying something before being mauled by a demon - but it was still unsettling and finally, Jace could see a way to make it better.

"Be my parabatai," he blurted out before he'd lost his nerve.

Alec froze on the spot.

"What?"

Jace gripped him by the hand as if the proposition would suddenly make more sense then. "Be my parabatai and neither of us will have to guess." It was a pleasant surprise that it sounded slightly more sensible now, at least to his own ears. "It doesn't have to be about loss if we choose each other."

"It's not the same," Alec protested, but the idea was just as appealing to him; Jace could see it.

"No, it's better. It'll mean our souls are one, won't it? It's just like—"

"Like a soulmate, yes." Incredulity and amusement fought for attention in Alec's voice. "We can't really cheat the system, though."

"We're Shadowhunters." He'd already swayed him, it would seem, but Jace wanted to make his point nevertheless. "If anyone can do it, it's us."

o.O.o

Meeting Clary Fray had shaken Jace's world in more ways than he could really define, but it was his understanding of soulmarks that suffered the most damage.

The spell that had hidden her heritage from her had hidden that too in an entirely new definition of what overkill meant and keeping them glamoured - an instinct for nearly every human or human-like being on Earth - was difficult at first. Jace hadn't even noticed until he'd seen her in the tiny dress she'd received from Izzy, but once he had, it was impossible to ignore.

She was covered in marks from her shoulders to her neckline and all the way down to both her wrists. She couldn't keep them invisible and they overrode the few runes she had so far; overrode everything about her.

"I know," Clary said as he stood frozen in front of her. "I have some down my back too, and over my stomach. How do you deal with this when you need new runes?" She grimaced at the thought. "Isn't it— distracting?"

"Most of us don't have more than two or three of them." Jace winced as soon as the words were out of his mouth. Way to make her feel even more abnormal than she already likely did. "It's— they're something we cherish."

"Can I see yours?"

Jace had never shown them to anyone, but this was safe, wasn't it? She was safe. And it was only fair, considering that he could see all of hers. "Sure."

Jace closed his eyes for long enough to bring them to the surface and knew that it had worked by Clary's sharp intake of breath and her touch on his arm. He peered at her suspiciously.

"Not what you expected?"

"Not really." Her eyes were fixed on the one near his shoulder – his soulmate's urging to look at them – and she had the distinct look of a deer caught in the headlights. "What about the white one?"

"That's from my mother, I think." Even now, after so many years, it was the only thing that made sense. "I don't know about the other two yet."

It was not a lie, exactly – the Be careful was written in a handwriting that had become all too familiar over time, but no one had to know that. Not so soon.

"I thought you said you lost your mother when you were a baby."

"It doesn't matter." Launching into an explanation of how soulmarks worked was easier than dealing with his own. "It's about the people your soul recognises as different from the others; special in a way that no one else is. Not every soulmate you have will have you as their own and vice versa. The ones who are a match are usually a darker colour, I've heard." Both of Jace's living soulmates were a match, so it was impossible for him to see the difference, but Clary nodded.

"I have two of those. And everyone else—"

"It just means you love easily. Don't sweat it." Jace patted her shoulder with a smile and handed her the blade he'd come to deliver in the first place. "Let's go."

o.O.o

By the time he witnessed his father fall to his knees and rise as Valentine Morgenstern, Jace was hanging on to the last shreds of his strength. He'd done everything, everything, to get the Cup and Clary's mother back. He'd ran from the Institute, had entered a different universe, had fought his own parabatai and it still led them all straight into Valentine's trap. He'd had enough and only a moment after that, when the attempt at a revelation from the man's side came, Jace snapped.

"You're not my father." He was, of course he was – he had been the one to raise him, after all – but not in the only way that mattered right now. "And Jocelyn is not my mother. My mother is dead. For all I know, you could have been the one to kill her."

Even with the Cup firmly in Clary's grasp, victory had never tasted sourer, but those weren't the news he brought home.

"You see," he told Alec late into the night when, after a messy reunion and days of silence, Izzy had finally managed to force them into the same space to talk, "soulmarks can be useful sometimes."

"They're also misleading," Alec insisted, stubborn as ever. "Take Simon and Clary. He died and came back to life and their soulmarks changed to their new last words to each other. None of it means anything where the Shadow World is involved."

What a rude thing to say to your soulmate, Jace almost admonished him, but the joke wasn't worth the infinitely worse prospect of finally admitting to either of them what he'd been painfully aware of for years. Did Alec know too? Had he realised? Jace would feel better if he had, for some reason he was afraid to define. He wouldn't feel so alone then. That way they could both comfortably cling to their bond in silence like they didn't need anything else.

"Don't be an asshole," he said instead, resting his head on Alec's shoulder, and sent a silent prayer to whatever higher being was currently watching that his last words etched into his parabatai's skin weren't in the same spirit.

o.O.o

It was much later, when Jace let himself be pulled through a portal to Idris with yet another of Alec's be carefuls in the background, that he realised with a sinking feeling that yes, this was it, this was the one. He managed a response somehow and almost laughed – he'd been right, after all. Alec wouldn't need to see him die. He seemed to have come to the same conclusion if the shout of 'Jace!' behind his back was anything to go by, but by then, they were already gone; Alec was saying it to the empty street all the way home in New York. So this was how it actually went. They hadn't even had the time to say a proper goodbye. Not that Jace had expected it – his mark told him that they wouldn't say another word to each other.

It was even later still when Jace managed to gasp out an I love you because Clary deserved to hear it, deserved to know that she wasn't alone, and he watched the words scar on the side of her neck as the stars blinked out one by one over his head.