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It was easier to agree to this when Izuku’s horns were just little nubs buried in his curls.
It was also easier to agree when Izuku was buried to the hilt in him and not moving. He was being an absolute bastard of a tease — if it weren’t in him, he’d call him his own cock-blocker — and staying perfectly still, not even letting Katsuki grind back against him for a hint of pleasure.
Instead, he kept him held in place, warm huffs of breath against his neck and his hand rubbing against his stomach, a little bulged from his cock.
“Kacchan,” he murmured, the motion of his petting just slightly too slow, making Katsuki squirm with anticipation. “Don’t you think it’d feel nice if you were stuffed a little more?”
“Isn't your cock enough?” Katsuki spat the words out, then bit his lip as Izuku’s fingers cupped just around where said cock sat inside of him. “Shut up and fuck me, bastard.”
If he turned to look at him — he would not — he would immediately recognize the expression on Izuku’s face. Curious, calculating, filled with wonder. Katsuki saw it often enough, and it was mostly directed straight at him. Izuku doing the mental math to see what Katsuki could do, how to push his limits. When they were younger, it was about his quirk. It still was today, but with the addition of Izuku playing with his body in bed.
“I think you could take it,” Izuku decides, Katsuki only has a second to think ‘What is it?’ before Izuku fucks into him, just a little excruciating pleasure, and Katsuki nearly weeps from it. Then he stills again, because he’s a fucking tease at the worst of times. “I bet you could take all of me, Kacchan. Don’t you think that would be fun?”
Katsuki’s brain was a little hazy from Izuku’s withholding of pleasure. Izuku knew all too well that denial was an easy way to get Katsuki more pliant and malleable — he’d tested every damned variable early in their relationship, and Katsuki’d nearly exploded him when he suggested that he abstain from touching his own goddamn pussy for the sake of the experiment — but that the best way was goading him.
He was being played like a fiddle, especially when —
“Unless you think you couldn’t take it, Kacchan…” Izuku demurred, before moving like he was about to pull out of Katsuki.
“Fucking fine,” Katsuki hissed. “Stuff me more or whatever you want, I can take it, I know I ca- ah!”
Delighted, Izuku immediately let his hand trail down to Katsuki’s clit and fucked into him, cutting off his words and punching the breath out of his lungs, keeping up a rapid pace with his hips as he brought Katsuki to a hasty, apologetic, grateful orgasm.
In the aftermath, Izuku rested his head on Katsuki’s chest, petting his stomach and eyeing it with a mix of hunger and longing. “Don’t worry Kacchan,” he’d murmured, when he thought Katsuki was asleep. “You’re gonna be such a good broodmare for me.”
And maybe it was Katsuki’s fault, just a little bit, for not asking what he meant by that, right then and there.
It’s not his fault that his partner was a goddamned dragon, okay?
Their next day off — after a relatively quiet week, minus a quirk accident that turned Shoto into a turtle for half a day, mostly filled with school visits and easy patrols — Katsuki’s dragons came to roost.
(Do dragons roost? He never thought to ask.)
Izuku told him to undress, that he’d get ready in the bathroom, that Katsuki should absolutely not come in no matter what he heard, and even though Katsuki was primed to disobey, he instead found himself waiting, feeling a little awkward despite the warmth of the room — the ever increasing warmth, because Izuku was definitely doing some dragon nonsense in there.
All that emerged through the closed door was a choked off scream and a low rumble, before it opened and Izuku stumbled out and —
“Why are you so big all of a sudden?” Katsuki blurted, eyes wide as he took in Izuku. Izuku, dragon lineage was usually limited to tiny horns buried in the mess of his hair and a resting body temp that meant Katsuki did not need to turn up the heat in winter. But somehow, he’d changed — body getting bigger, horns lengthening until they were just the right kind of grabbable, nails thickening to claws. In his sheepish smile, he saw a glimmer of a fang, and green scales painted their way down his neck and chest, burying themselves at the base of his cock. His much bigger, thicker, ridged cock, covered in those same green scales, with a head that would surely bruise him open. “Have you been able to do this the whole time we've been together?”
Izuku blushed, skin turning a little green, but Katsuki’s eyes were glued to his cock.
“It's my mating form…” he replied, before he scented the air and smelt something familiar and human — slick and musk, right between Katsuki's thighs. “You're already ready for me, Kacchan?” His voice was full of delight as he stalked towards Katsuki, who tried to stammer and protest but couldn't deny the wet patch forming between his thighs.
“It's not — it’s just — mating form?” Katsuki said breathlessly, before licking his lips as Izuku’s cock hardened, just from the smell of him.
“I get it, I get it,” Izuku said, his brain clouding with lay lay lay as he looked over Katsuki's pale stomach, so flat. As he thought about filling it up with his eggs, the ones that need a cooler body than his to mature. “You just want to take my eggs, isn't that right, Kacchan?”
Katsuki blushed but didn't reply, his legs widening instinctually as Izuku started to loom over him on their bed, crawling between them as his cock — huge, lurid, green and ridged and so much bigger than it usually was, how dare he hold this back from him when there been fucking for years — brushed against his inner thighs and Katsuki shivered.
“What do you mean by eggs?” Katsuki said, his voice low. Izuku was bigger than him like this, all around. Stronger arms pinned him to the bed, bigger eyes blinked at him in wonder and awe and heat. There's an inner eyelid that he’d never noticed, or maybe it only appeared when he transformed; it blinked sideways, taking a second longer than his human ones to open, and the effect should be unsettling.
Should make him feel like he’s in danger.
Should activate some hidden part of his lizard brain, the part that screamed flee when he was matched up against a villain he couldn’t possibly take on alone.
But Katsuki was never one to flee; he’d always choose to fight in the face of danger instead.
Against this unknown? He’d choose to fuck instead. The third base instinct.
“You're gonna be so good for me, Kacchan,” Izuku murmured, so close to his face, long lizard tongue slipping out to lick against his neck, taste his sweat; a mix of fear and pleasure. “Gonna take my eggs so well. Keep them nice and safe for when they hatch.”
“H-how long will that take?” Katsuki asked — something else he should have questioned earlier, because they only have one single day off — but then Izuku’s cock started to twitch against his pussy, so wet, so wanting.
“Don’t worry, you’ll be so good for me,” he repeated, “so good…”
And before Katsuki could say you didn’t answer the question, jackass, or realize that Izuku was quickly losing himself to some kind of mating haze, the tip of his cock found a space between his lips and pushed.
The pressure, for a second, wasn’t too bad. Izuku's cock was warm, almost comforting, against the muscle. It was pleasurable, just a little too big, giving Katsuki the right kind of burn, and he pushed his hips against it to get more until he felt the dull push of the first ridge of Izuku’s cock against him.
“Fuck,” Katsuki gasped, struggling against the sensation, so much wider than he'd expected. “It’s not gonna fit, Izuku, it’s not—”
“Shh, easy Kacchan, easy. You’re going to take it, don’t worry” Izuku cooed, and it's not calming, it’s not an answer. It felt like he was being split apart as the cock pushed deeper into him, so impossibly wide. He had a brief fear that he’d be torn in half as Izuku licked up more of his sweat, and groaned with a dragon-deep rumble as he pushed in.
Katsuki’s mind was a litany of curses — an invocation more than anything, because all of a sudden it was in, wedged into his pussy and spreading him wider than he’d ever been before. His bastard of a dragon's hips jammed deeper between his legs, to the next ridge, because he’d used too much force pushing the first one in, and Katsuki let out an honest-to-god whine.
And the eggs hadn’t even come in.
“Just a couple more,” Izuku lied, because Katsuki knew exactly how many ridges were on that cock. The image of it was fixed in his mind, and he could imagine what it looked like — each ridge pressing against him, the head lodging itself deeper and deeper into him, kissing places within him he’d never thought reachable.
Each next push felt like it took an eternity, and the cock got thicker towards the base, because Izuku couldn’t make being a dragon easy on Katsuki, could he? Each time he tried to squirm away from the sensation, Izuku just pulled him back until he braced a forearm against his chest to keep him pinned.
The tears that emerge with each thrust, each new crest tugging at his walls, spreading him wider than he could have thought possible, are cleaned up by Izuku and his tongue. And when he started to curse Izuku for keeping this from him, Izuku buried that tongue in his mouth to quiet him.
But then, miraculously, he feels his ass reach Izuku’s thighs, and realizes — even though most of Izuku’s body is blocking it — that, like a miracle, he’s bottomed out inside of him, and it almost feels —
Now that he’s used to it —
Good.
They paused like that for a moment, Izuku breathing heavily, those rumbles vibrating through Katsuki’s body and his pussy as he caught his breath. He could even squirm in place a little and feel those ridges press against different parts of him, gasping as his core tightened involuntarily at the little shocks of pleasure.
He felt… weightless.
“That's all?” Katsuki started to taunt, forgetting, for a moment, that the eye of the storm is the calmest.
“So close,” Izuku breathed out, as an unfamiliar heat flared inside of him, unexpectedly, and -
Izuku pulled him up, rising to his knees and turning him around on his cock to seat him on his lap. He pressed a hand against his belly, feeling the bulge of his cock inside of Katsuki. Now that he could see more, Katsuki can tell just how much bigger Izuku’s cock is, compared to when he’d agreed to this.
Katsuki had a moment to curse him and protest — from the way the motions had very nearly coaxed an orgasm from him — before he started to feel movement.
Right.
The eggs.
It started low, at that point where their bodies were pressed together. He let out a shivery hiss as it moved through his cunt, trying and failing to flee from the feeling because Izuku just held him tighter, thick canines against his neck as he craned his head to watch those eggs flow into Katsuki.
Izuku groaned, hips working deep into him, core tight to push them out, to lay them inside of Katsuki.
All he could do was lay a hand on his stomach, and Izuku let him slip it underneath his own larger hand, the claws little pinpricks against Katsuki’s wrist.
The eggs were curiously soft, nothing at all like a chicken's egg, and though they were big they still seemed willing to bend to the shape of Izuku’s cock — ovipositor? — and find a place inside of Katsuku.
He could see the trail as they traveled through his body, skin bulging in places along the cock. Could feel it when they passed under where his hand was, distending under his fingertips. And when they erupted from the head of Izuku’s cock, it felt hot. Hotter than Izuku’s cum, and half as fluid, and behind him Izuku would moan out his name as each egg of the clutch was laid and found a home in Katsuki’s body.
More than leaving a trail of movement, they were stretching his stomach out; swelling it, thickening it. Rounding it, until Izuku was petting against the curve of it and he couldn’t make out his thighs anymore, the view blocked by the bulge of the eggs inside of him.
He felt outside of his own body even as his pleasure burned slowly within him. It's nothing like Izuku cumming inside of him; it's better somehow, something solid left behind. And the slow-burning orgasm inside of him had roots in the trust flowing from Izuku, to be his broodmare, to be a safe place for these eggs to rest. In knowing that some part of them would live on in whichever survived from the clutch.
Katsuki had a brief vision of little dragon babies, small blonds with oversized wings and horns, skin glittering with explosions, before the final egg latched itself inside of him.
“Just one more step,” Izuku groaned, pulling out of Katsuki and laying him down, leaving him curiously empty before the space was filled by the eggs moving into place. He probably should have been more curious, but all he could think about was how big he was, how exhausted he was, how he’d probably waddle when he walked — if he could even walk, because his legs felt like jelly — and how he hadn’t even fucking come.
He was so distracted that he didn’t notice Izuku moving to crawl between his thighs, and his belly hid most of him except the very tip of his horns.
Katsuki blinked curiously down at him, thoughts coming a little slow, before he felt a warm huff of breath against his pussy and —
Fucking dragons.
That long, muscular tongue pressed against his swollen, aching entrance, toying with his fat lips and his clit before pressing inside of him. Izuku’s big hands held his hips in place while Katsuki bucked, trying to grind down on his face.
All of those eggs, and Izuku still had the energy to eat him out? Maybe it was Katsuki’s time to get curious, and see what all that dragon stamina could do.
He licked and sucked and salivated over Katsuki’s cunt, murmuring praise to Katsuki and sweet promises to the eggs inside of him. His thighs tightened, his body feeling electric, and he could tell he was close, toes starting to curl and dig into the mattress as he cursed and gasped.
Katsuki reached down to grab at Izuku’s horns, to get his bearings and hold tight to something, but he couldn’t reach. Not with his stomach so round, so bulged, and he tossed his head back onto the pillow and nearly wept until Izuku’s hand entwined with his own, almost too big for comfort, but giving Katsuki something to hold fast to as he came, finally, agonizingly, on Izuku’s cursed and talented and divine tongue.
Izuku licked his leaking slick through his orgasm, and Katsuki felt his core tighten in new ways to keep the eggs from coming out with it — he needed to be good, to keep them safe until it was time to lay.
And when Izuku finally rose, face still wet from Katsuki’s pleasure, looking rather like the cat that got the cream, Katsuki groaned, trying to wipe the sweat from his brow.
“When do these come out?” he muttered, already turning on his side so Izuku could spoon him, and cradle his stomach while they rested.
“When they’re good and ready, Kacchan,” Izuku replied, already falling into an easy sleep, even while his cock brushed against Katsuki’s thighs and made him shiver.
When he finally laid the clutch, two days later after an awkward call to Half-and-half to beg him to take on a few extra shifts, Izuku coaxing them out of his body and praising Katsuki while he cursed his dragon to the high heavens, he couldn’t hide his disappointment that the eggs were inert.
“All that for nothing?” He’d felt almost murderous looking at the remnants of the clutch, and Izuku — back in his usual, more human form — rubbed his head, apologetic.
“I guess — I mean — do you want eggs that work? There’s never been a — our bodies aren’t compatible like that, I thought, but I guess we could try?”
That sparkle of wonder filled Izuku’s eyes again, and he pulled out a new notebook to start recording his thoughts about how exactly they could get viable eggs, and Katsuki grinned.
It would require a lot of experimentation, he was sure, as he rubbed his stomach.
He couldn’t wait for it to get big again.
And again, and again, and again.