A/N: So… I really shouldn't be doing this. I have a Twilight fanfic that desperately needs to be updated, and an original novel that is more than halfway complete, and here I am writing Mortal Instruments Fanfic. Why? Because I'm addicted. And I need more. And there's just not enough out there.

Summary: She knew what this was. This was about war. This was about hope. This was push and pull, fall and fly, live and die. This was survival. And when you're only trying to survive… anything goes.

WARNING: This work is rated M, for sexual themes, and (possible) incest. If that bothers you, turn around now. Also, I do not own Mortal Instruments, I'm merely borrowing Cassie's characters and using them as my own little playthings.

Survival

Clary shivered violently against the cold wall of the cave. She could feel the jagged rock jutting into her back, scratching her frozen skin, but she hadn't the will to move or the desire to care. Her jaw ached from the force with which she clenched it. She refused to let her teeth chatter. Because with each raw clacking noise, she saw the toll it took on Jace. How much it hurt him that the two of them were stuck here, waiting to die, or worse.

For him, she still did have the energy to care. So she screwed her mouth firmly shut, until she thought her gums might bleed or her teeth grind into dust.

In the beginning, they'd both thrown all of their energy into fighting the land, fighting the weather—searching for a way to escape this place—wherever they were. Stranded high in some vast range of mountains where there was no one to hear them scream, no one to guide them to safety, no one to find their bones frozen forever in the snow.

Maybe that was the way Valentine intended it. At least, that was what Clary thought. Jace refused to believe it. He swore that of all the vile things his father might be, he was above all selfish. And Valentine wanted them—his creations, his pawns. He wouldn't leave them to die.

Clary agreed. But she wondered if perhaps they had gone too far. Even Valentine must have some breaking point where their potential value was outweighed by the harm they were causing to his plans. Perhaps he just meant to get them out of the way for a while, until he succeeded. Or perhaps they were meant to die here.

They'd moved swiftly those first few days, even journeying through the night by witchlight.

Between the two of them they were left with two seraph blades, a dagger, and a bow. Jace used the dagger and bow to kill game, but he kept the seraph blades safely in his belt. Clary wondered what he was saving them for—what he expected to run into out in this wilderness.

On the fifth day, they'd found a cave, tucked away into the side of the rock, deep enough to shield them from the snow and wind, and dry enough that they could build a fire. They'd only intended to rest for the night, then keep moving in the morning, but when Clary woke her skin felt tight and blistered against her bones. Her body ached, and she could feel the ice creeping in through every pore.

She'd never seen Jace look so angry, so ready to kill. His expression made her heart clench, but she knew he wasn't angry with her. Jace wasn't used to worrying about other people. He wasn't used to being scared of death. And above all else, he wasn't used to failing. And he had failed her, or so he thought.

He was wearing next to nothing because he'd covered her in as many layers as he could. He periodically ventured out into the heavy winter storms to find more firewood. He was like a machine, never mindful of his own pain, his own discomfort—working only for her. For them. For survival.

She drifted in and out of consciousness, her hazy thoughts pierced occasionally with the random thought of the war that was currently occurring without them, the safety of their friends, the way she still couldn't keep her eyes off of Jace's bare arms even when her veins felt frozen and brittle.

She didn't realize how serious things had become until she woke in the middle of the night to a sound so heart wrenching that she felt her spine twist with the realization.

Jace was huddled over a dead fire, striking rock against rock with such desperation that she could feel it pouring out of him in waves. There was a low, keening cry that pricked the silence with each strike of the rock, each determined breath, each self-loathing curse that slipped from his mouth.

"J-Jace" She loathed the weakness of her voice, the stuttering over her words.

He was at her side at an instant, and it took her eyes several long moments, longer than it should have, to adjust to be able to see him properly. There were black half-moons under his bloodshot eyes, his cheeks looked somehow hollow, but he was still beautiful. He was still Jace. He looked like an angel trying to carry the whole world's grief on his wingless shoulders.

His cold hands touch her face, and she leaned into him.

"Christ." He murmured under his breath. "You're burning up, Clary."

Her teeth clattered loudly, before she could get them under control. His eyes darkened to a molten gold. She saw them shift. An idea formed, and a decision was made.

And then he kissed her.

And it was so unlike any of their other kisses. It was fragile, but rough—and edge to it that wasn't about love or lust or passion.

She knew what this was. This was about war. This was about hope. This was push and pull, fall and fly, live and die. This was survival. And when you're only trying to survive… anything goes.

His tongue dipped into her mouth, swirling with her own, and she shivered at the heat. His eyes begged for permission that he didn't even need. She pulled him down against her, her hands on his warm skin, wanting to be closer.

He pulled back her blanket, and even though she had several more layers, she swore she could feel the heat of his hands.

She hurriedly undid the buttons of his coat, trying unsuccessfully to wriggle her way out of it.

He chuckled against her lips. "Impatient?"

"Very." She grinned.

His eyes no longer held the shame and desperation from earlier. They were infused with a churning heat that sent a tingle up her lower back. He removed each layer carefully, with such precision that she wondered if he expected her to break.

Finally she was left in nothing but a t-shirt and jeans, and she could feel the chill of the air nipping at her skin. He stopped again, and now he looked nervous. It was such an odd emotion to see from him that she laughed aloud.

"Really? You're laughing at me?" He asked incredulously with just a hint of a smile around the corners of his mouth.

"In that case." He pushed himself up onto the balls of his feet, and made to retreat.

"No!" Clary cried, grabbing him by his thin undershirt, and pulling him flush against her chest. She felt the air rush from her lungs and belatedly added, "Don't stop."

The playfulness left Jace's demeanor, to be replaced by a dark expression, a want so strong it bordered on hunger. He felt those words reverberate in his mind and slither down his spine.

Don't stop.

He didn't think he could have if he tried.

His body was flush against hers. He could feel the swell of her breasts pressed against him, and his hips were fitted in between her open thighs. She wrapped her arms around his middle, and in a display of daring that surprised him, her small hands slipped under his shirt. Her little fingers splayed across his lower back. He groaned at her touch, and dropped his face into the crook of her neck.

He was getting distracted. His whole intention (or rather his excuse to touch her) was to keep her warm. And he knew the best way to do that.

He sat back again on his knees, and she groaned at the loss. He smiled widely as her arms instinctively reached for him. How had he gone this long without touching her? Without kissing those lips? Sister, be damned.

He loved her.

And he couldn't live without her.

And that's all it really came down to.

If he lost her because of Valentine, or his own inability to take care of her—he wouldn't survive it.

Survive.

That put a new urgency in his actions, and he pulled his shirt off as quickly as he could. He felt his hands shake with nerves again as he moved to the button of his jeans. But now wasn't the time to be modest. There were more important things.

He looked back to Clary, surprised to see that she had removed her shirt, and had her arms covering her bare chest, shivering.

"Oh. Clary." He rubbed his hands up and down her arms, trying to create some friction.

But it appeared Clary was interested in a different kind of friction. She hooked her legs around his hips, and pulled him down towards her.

He laughed, but obliged.

He ran his fingers along the top of her jeans, skimming the skin underneath.

"We should, uh, get these off first."

She nodded, her hands still crossed over her chest.

Jace removed her pants slowly, trying not to let her see the way his fingers fumbled. Because this didn't seem real. He'd dreamed about moments like this—always too afraid to make them come true.

When he was left only in boxers, and she in a pair of sensible bikini-cut Hanes underwear. He grabbed the blankets and pulled them snuggly around them, wrapping them up as best he could.

"Are you okay, Fray?" he asked, hovering over her.

She rolled her eyes at the name, but nodded.

"Is this okay?" He replied hesitantly.

She met his tawny gaze, saw only reassurance and love in their depths, and nodded in affirmation.

He leaned down, and placed a tentative kiss on her lips.

"Are you feeling warmer?"

She looked at him through heavy-lidded eyes.

"Yes."

He kissed her again, deeper this time. And he felt something well up within him. The saddest kind of happiness or the happiest kind of sadness. Her tongue glided against his own, and he had to fight off the self-loathing—why had he been so stupid? How much time had he wasted doing the right thing?

He kissed a trail down the side of her neck, lingering over her pulse point. She smelled like home—she smelled like Clary.

Her eyes were closed tight. She thought she might die from overstimulation. His lips skimmed down her shoulder, and he sowed slow, hot kisses along the arms covering her chest. He looked up at her from his position, and a blush broke out across her face and neck.

His fingers caressed her arms slowly, lovingly. He watched her to make sure he wasn't scaring her, and then slowly began to peel her arms away. He wanted to stop and drink up his fill of her. He wanted to study every nuance of her body, but he knew she was self conscious, and he also knew that she needed body heat. He went back to kissing her neck, his tongue darting out to trace designs on her skin.

And then he lowered himself until they were bare chest pressed against bare chest. Clary gasped at the feeling. Her body felt like it was melting, and she suddenly had to kiss him.

She placed a hand on each side of his face and pulled him up to her level. His arms were wrapped around her tightly, their legs intertwined, and their lips crashed together like waves at the tide. His tongue plundered her mouth with an urgency that she matched. She tangled one hand in his perfect blond hair, and the other began to trace the divots of muscle on his back. He groaned low in his throat, and then she felt the heat of him, long and hard, pressed against her thigh.

She swallowed back her nervousness, losing herself in his kiss. When he nipped at her lips lightly, she automatically lifted her hips, grinding against him.

He broke the kiss, panting against her neck. He stayed there raining small kisses along her small shoulders, and then gently, meticulously pressed his pelvis against hers. She rocked up to meet him, her eyes scrunched at the feeling of pleasure pooling deep in her core.

She knew this was about survival. She didn't doubt that he had feelings for her or was attracted to her. But she also knew that Jace was strong. And the only reason he was doing this now was because he absolutely had no other choice.

But it was about survival for her too.

And when it comes to surviving… anything goes.

She placed a hand on his shoulder, and tenderly pushed him back. He parted from her luscious skin reluctantly, and for a moment he thought she was going to call the whole thing off. And the way his heart seemed to shrivel in his chest for the barest moments told he that not only would he not survive without Clary. He couldn't survive without loving Clary—every day, every moment for the rest of his life.

But then her hands slipped down between them, and she started wriggling out of her last piece of clothing.

He thought he might be blushing. HE, Jace Wayland… blushing. So he quickly ducked his head, and took the opportunity to follow her lead.

What followed was surprisingly not at all awkward. Rather it felt like the most natural thing in the world. It was like fitting together two missing pieces from a puzzle left uncompleted for years. It was the end of something that had begun long ago.

And it was the beginning of something entirely different.

It hurt, at first, for Clary. But Jace was gentle, and refused to move until she had adjusted to the feeling of them being joined. Of him filling her in a way that neither ever thought would come to pass.

"I love you." He whispered, and it echoed off the walls of the cave.

She smiled up at him.

"Are you just saying that to get in my pants?" She whispered back.

He smirked and brushed a lingering kiss on her lips.

"I believe I'm already in your pants, Clarissa."

She returned his smirk, which widened into a devilish grin as she shifted her hips upward.

His eyes fell shut, and a low moan rumbled in his chest.

"Minx." He muttered, and without waiting for her laughter to finish, he pulled out, and then thrust back into her heat.

Her mouth fell open in a silent scream.

He repeated the motion, and her legs came to wrap around his lower back to hold him close and use as leverage.

"What, no sly comment?"

Her face was a mask of bliss, and she pinched his forearm in response.

"Ow!" He responded by rocking his hips roughly against her own. "Who knew little Clary like to play rough."

She pinched him again, this time dangerously close to his nipple.

He hissed, then leered at her. "Don't worry. Jace likes to play rough too."

She laughed and swatted him on the head, "You incorrigible."

"But you love me."

She looked at him seriously then, and their movements slowed to something sweet and tortuous.

"I do." She promised.

"I know."

And they both knew what this was about. It was about war. And hope. It was about push and pull, fall and fly, live and die.

It had everything to do with survival.

And maybe… just maybe… it had even more to do with love.