AN:- Warnings for dark, creepy fic. Written for the prompt: Cloud/Zack: Not human - experiments gone wrong.

The first time Cloud tells him that his eyes glow in the dark, Zack is up off the bed and stumbling his way to the mirror before the youth can get out that it's a compliment. The exclamations of delight from the other side of the room tell him the words aren't necessary after all.

The rest of the changes come more slowly, more subtly.

Cloud would say more insidiously, but he doesn't want to sound like he's scared or anything, and Zack? Zack's so confident about it all that everything must be okay. Besides, he never stops grinning and teasing and playing, quick touches and words that mean more if you know what he's really referring to. Cloud blushes and stutters and forgets and days turn into weeks and weeks into months and then, one day-

Zack's back from the labs and he's grinning with an edge that Cloud's never seen before. He's high and happy and laughing, and Cloud can't quite put his finger on why, but somehow it doesn't feel quite right.

Zack drags him out the door and down into town, into the slums and the seedy bars they've never been to before. Cloud's furious, because pretty much all these places are off-limits to a recruit like him and he's terrified one of the ranking officers or worse, one of the other SOLDIERS (because Shiva knows, the SOLDIERS gossip like fishwives) will see them and report him. But Zack's strong, so damned strong, and he's quick and good at herding him and Cloud's not exactly going to stand in the street and scream like a little kid to be let go.

Later on when Zack caves and lets Cloud drag them both home (and Cloud had to be clever to encourage him to leave so early, sex sells after all) they reach the barracks and Cloud tries to close the door on him he's so angry. But Zack's quicker, always quicker.

"What's wrong, babe? C'mon, you promised...it's okay, nothing happened. No-one saw us..."

And even though he's angry, Cloud's thrilled too because the people in the bars looked at him and saw Zack's arms round his shoulders and Zack's hands on his waist and that felt good. So he only fights a little, for the principle of the thing, before he lets Zack have his way.

The next morning Zack's full of apologies and shame. Cloud hears excuses of mako and shots, promises that it'll never happen again, that it'll never go that far again. Sorry, babe, so sorry. It's just the mako, you know? You'll understand one day.

And for a while it's good, except Zack's stronger these days, much stronger. Cloud can see it in the way he moves, the way he trains, the way he lifts his weapons like they're weightless. (Cloud tries to pick one up himself once when Zack's in the shower and it's a cold shock when he realises he can just about lift the thing with both hands. He's quiet the rest of the evening and not even Zack's most cunning kisses can wheedle the reason why out of him.)

The next time Zack gets a shot he comes back laughing and drunk on mako, and when he drags Cloud down to the floor of his apartment there's no fighting him. It's good sex, hot and hard yet still tempered with an obvious care for his ordinary human body, but afterwards Cloud feels furious with Zack for not giving him a choice and disgusted with himself for giving in so easily.

Zack goes out afterwards and doesn't return until morning. Cloud's already up when he gets back and finds that he just can't bring himself to speak to the other man, even when Zack creeps up behind him to slip his arms around his waist, rest his head on his shoulder and whisper sorry, babe into Cloud's hair.

He's gone on a mission for a week after that which gives Cloud the space to sit back and think things over. He takes his time, driving himself into the training sessions and practice manoeuvres his squad goes on and the physical activity clears his head. By the time the week's up and Zack's due back, Cloud's convinced he's overreacting. Zack's a SOLDIER, and that means he has to have an edge that other men don't have. It's stupid for Cloud to react the way he is, like a scared little kid terrified of playing with the older boys. And damn it but Cloud's going to be a SOLDIER too one day, so rather than whining he's going to be the strong right hand Zack needs him to be. Zack needs to be able to rely on him.

When Zack gets back the grin he gives Cloud tells the youth everything's okay again.

Two mako shots later and it's not just the day after the injection that Zack spends laughing and strange. He's high and crazy for two days, three, and then more. A week. The fifth shot comes nine days after the fourth (Cloud knows because now he's counting) and Zack tells him square to be out when he gets back. Cloud takes the advice and spends the evening in his own dorm, something he hasn't done for weeks now. The other recruits know he's bunking with a SOLDIER but this is ShinRa and in this company you take every advantage you can get. Still, that's why he's in his own room and not keeping company with someone else tonight because still, he is screwing a SOLDIER.

Zack turns up again four days later when Cloud's eating dinner at a corner table in the canteen. The SOLDIER's skin is pale, his eyes glassy and the hand that holds the Styrofoam cup of coffee shakes just a little. Cloud eyes him quietly and doesn't ask. Later they go back to Zack's rooms and Cloud lets himself be stripped and touched and kissed until neither of them can breathe. He comes first, mouth open, a half-curse dying on his lips as he shudders to completion.

Zack follows him soon after, burying his nose in the crook of Cloud's shoulder and growling low as he arches into a final thrust. Cloud lets him lie there on top of him afterwards, even though it makes it hard to breathe, and strokes his fingers across the back of the other man's neck. Slow, soothing caresses that lull the SOLDIER into sleep.

Later, Cloud goes to shower. He has to catch himself and reach clumsily for the edge of the sink to stop himself falling when the sight of the blood coating the inside of the bath makes him jump. He leans forward carefully, not really believing his eyes, his brain searching for another explanation. It's dye right? Hair dye?

He cleans the bath carefully, too embarrassed to leave it in that state but worried too that Zack will somehow be angry with him if he does. A moment's thought tells him that Zack most likely never intended for him to see this and it's just better if he pretends it was never even there.

Zack never mentions it and neither does Cloud. After that though, Cloud makes certain to be out on shot nights.

Zack never talks about the shots and Cloud tries not to ask. It's not until the tenth, eleventh and twelfth shots that just staying out of the way ceases to be an option. This time it's six days after the last shot and Zack comes to find him. He stands beneath Cloud's first floor window and throws stones up at the glass until Cloud moves to the window to find out who the joker is. A cold shock goes through him when he sees the eyes glowing up at him from the darkness and for a moment he thinks one of the monsters has somehow made it onto the camp before he realises just who it is.

"Zack?" he asks, leaning out the window for a better look.

And that's when the other man leaps. Cloud's stumbling backwards in shock, falling on his backside as Zack appears in the open window, shoulders hunched, head cocked and eyes glowing balefully.

"Fuck..." Cloud gasps. "How the hell did you...?"

"Not bad, huh?" Zack drawls, and his voice is lazy but his grin has a razor edge. "Let's go have some fun."

"No."

There's no way in hell Cloud's going with him when he's like this. He's seen this before and last time it ended in bloodstains in the bath and a silence that made him nervous for a week afterwards.

Zack stares at him and the angle of his head makes Cloud think of a bird, or a lizard. Something cold and predatory. He stares for a long time and suddenly Cloud realises that his breath is coming in short, rapid bursts, like he can't get it under control. Suddenly Zack shrugs and looks away. "Okay," he says and as quick as that the window is empty again and he is gone.

Cloud stumbles over to pull the pane closed and draw the curtains. He doesn't take long to look, but it's enough to tell him that Zack is already long gone.

It takes a long time to get to sleep that night.

In some countries, thirteen's an unlucky number. Both Zack and Cloud agree it probably is when the thirteenth shot goes to hell. Cloud doesn't get any warning for this one and when Zack stumbles in the door to his apartment, Cloud's still there. He freezes when he sees the other man and Zack looks up at him with bleary, unfocussed eyes. His skin is pulled tight across his features and a sheen of sweat coats his skin.

"Hey, babe," he says. "Don't, don't feel too good."

He hits the floor with a crack of bone on thin carpet and Cloud drops the sandwich he's making half on the plate and half on the floor as he rushes to his side.

"What happened?" he demands and Zack laughs strangely and waves a hand in the air.

"Got a shot," he replies. "Doesn't feel good..."

It's a bad night for both of them. Zack's in pain and there's blood, from his ears and his nose and his eyes, but he won't let Cloud call anyone in to help. Cloud wipes it away with toilet paper, his hands shaking so badly he almost jabs Zack in the eyes more than once. There comes a point when Zack's moaning and clutching his stomach, his eyes glowing so bright and his skin so hot it almost burns that Cloud decides enough is enough and makes for the door.

"I'm going to get help," he says, crossing the room. "I'll be back soo-"

Zack moves impossibly fast, faster than anything Cloud has ever seen before, and the door that he's gotten half-open is ripped out of his hands and slammed closed.

"No," Zack grates. "No help."

Cloud stares at him in horror, at the heaving chest and the sweat that drools off of him and the look in his eyes that makes his instincts scream RUN! Very, very slowly he reaches out and places a hand on the top of his messy hair.

"Okay," he whispers. "Whatever you want."

He manages to get them both to the bed, which is a battle because Zack's heavy, and there they stay for the rest of the night. Zack's fever breaks some time just before dawn and finally he falls into sleep leaving Cloud stroking his shoulders absently and staring at the balled-up tissues until full daylight pours through the windows again.

"Sorry, babe," Zack says. "I didn't mean for you to have to go through that. It's all part of the programme though, and it's cutting-edge you know? Sometimes we have to take the ups with the downs. In the end, it'll be okay, you'll see. And then, by the time you're a SOLDIER, all these glitches will be ironed out. Everything will be fine."

He grins that infectious grin of his and pulls Cloud in for a kiss that steals all his fears away.

Cloud's not sure what to make of it all really. The blood scared him, freaked him out even. No-one should bleed through the skin like that, not without good reason. But then he looks at Zack and watches him train and grin and tease, sees him lift his sword like it's made of air and he thinks to himself maybe it's worth it, to be like that.

And if sometimes Zack looks a little odd, or stares at things Cloud can't see, or if sometimes there's a little bit of pain, well. Surely it'll all be worth it in the end.