This is based on the song Carnivore by Starset and vaguely by a friend's fic.
I'm pretty proud of the emotions it invokes so I hope you like it :)

Some might say Fate is cruel. Fate isn't cruel. Fate is neither good nor bad, she merely tries to keep the world running. It is subverting Fate that brings ruin.

Gray and Natsu had met as children. Two boys, polar opposites but similar in so many ways. As they got older, their Destiny became revealed to them. They would be the key to defeating Zeref; the Black Mage who threatened the world.

It was a big weight on the shoulders on young boys with difficult histories. But at least they were in it together. They took comfort in this, for although they fought, they trusted one another completely.

More years, more training, more threats. Lives intertwined always leads to something more. Friends became lovers, finding solace in intimacy, binding them closer. And making them better fighters.

But then it went wrong. They were still too young when Zeref decided to change Fate. Natsu was taken, his past revealed as their enemy's younger brother. He didn't go without a fight, but in the end he was overwhelmed. Gray's world collapsed as the one he loved, the one person he relied on to always be there, disappeared without a trace.

Zeref succeeded. The course of history was changed. Fate is not meant to be ignored. She fights back to return the world to its natural state. Chaos fell over the land.

Gray's training was not stopped. If the Black Mage could alter what was to be, then they would match it. Surely, as long as they still had one of the original pair, the Destiny could still be fulfilled. He threw himself into it. He would avenge Natsu. He would destroy that which took everything from him.

War came. The armies of Good and Evil clashed on the battlefield. Gray was now an adult. He stood on the front lines and fought. He tore down those stood in his way. The Elders murmured, worried for his mind. But he was their hope; they could not afford to remove their support now. The risk was waved aside. He would be fine. He would win. He had to.

Days came and went like every other. Neither side giving ground, no victor in sight. Then the game changed again.

Gray stood on the battlefield, face to face with the one person he never thought he would see again. Natsu. He had aged like Gray, his hair a little longer, but he was also no longer human. Horns curled towards the sky from each side of his head, deep red scales framed his face and crept down his arms, his fingernails were now sharp claws. And he was looking at Gray with recognition.

Gray stared in shock. Here was the man he had been fighting for all his life. Alive, contrary to what he had believed. But disdain flashed through Natsu's eyes and Gray's emotions quickly turned to anger. Anger at Zeref and his people for doing this, anger at his Elders for not doing more, anger at Natsu for not coming back, and anger at himself for not finding him sooner.

With matching roars, they clashed on the field. Once upon a time they believed they only had each other. Now they were forced to fight. And chances were only one would come out alive.

Every time they fought, Gray's very soul felt torn to shreds. He tried to talk to Narsu only to find him a warped version of who Gray remembered. But underneath it all, there was someone Gray knew. The boy he remembered; quick to grin, enthusiastic, smart, teasing. It was still there and he found himself just wanting him to come back.

The land the war raged on became destroyed. It cracked and split from the tremors of the opposing forces meeting. Mountains and ravines came and went in mere days. It wasn't just the armies that made it a dangerous place to be.

Parrying a lunging attack from Natsu found them both tumbling into one of these ravines, separating them from the rest of the world. It was a long, hard fall and both lay for a moment at the bottom, recovering. Their eyes met across only a few metres, trying to gauge the next play.

Gray wasn't sure who moved first, but he didn't think either of them expected how their bodies reacted. Self control loosened from the effects of the fall, clothing was torn off and thrown aside as bodies collided in a different way.

There is a fine line between love and hate. Passion can mean both anger and lust.

Gray cried out as his back hit a rock wall and Natsu drove into him. He clawed at Natsu's back, pulled at his hair, gripped his horns. Natsu pinned him, bruising him, biting him, marking him. Gray craved it. Sought the pain and the pleasure all in one. He couldn't help but want this; the memory of happier times still haunted him as he tried to regain what he had lost all those years ago.

It became a regular occurrence. During the day, the two would meet on the battlefield and seek to destroy. At night, they would sneak away from their respective camps to meet in secret.

Gray found himself tormented. Every night he saw Natsu. Every night he hated himself more for not being strong enough to resist.

Whatever desperate, temporary hope Gray felt, it was clear was different for Natsu. He was a demon now. Completely. He whispered things to Gray as he hurt him, as he kissed him better, finding sick amusement in Gray's distress. Natsu didn't entirely understand his own obsession with the dark haired man, but he wanted to keep him by his side forever.

So he asked. Asked for everything. And Gray offered it willingly. In the throes of ecstasy and agony, Gray begged Natsu to ease his nightmare, to take everything he was. His innermost desires were choked out.

Make him fall. Make him bleed. Make the horror stop.

Let us be together again.

Change me.

He didn't notice as his sanity was worn away.

One morning the camp that fought for Fate woke up to find their hope missing. Their general, their weapon, was nowhere to be found. They worried, scared that he had been injured during the night or there had been a sneak attack. But they could not dawdle long. The other army gathered, ready to begin the new day of death.

The fears of Gray's physical health disappeared in the face of a worse reality than they could have ever imagined.

He was already on the battlefield. Standing against them.

He stood tall at the side of the demon that was once Natsu. Gray's eyes were glazed and wild; insanity burning beyond control within his heart. Twisted by love that was turned against him. Forced into a corner by powers beyond his control.

The demon grinned, putting an arm around Gray's waist and nuzzling into his neck. He cackled at the expressions of the Elders as he ran his tongue along his lover's jawline.

Etherious Natsu Dragneel had won. He had claimed his prize.

Hope was lost. Together the pair were unstoppable. The army that believed they had Fate on their side was obliterated. The stories were wrong. Good didn't always win.

But Fate knows what she's doing.

The war over and together at last, the pair turned on their master. Zeref was destroyed and Fate regained control.

Despite peace returning, it had come at great cost.

Fate is not cruel, but she does not care for her pawns.

Two boys had lost their humanity, never to be saved.