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He’s tied up and helpless, his arms trapped behind his back in this alleyway, ropes knotted too tight to move no matter how hard he struggles. If Muzan trusts one thing, it’s his ability to tie a knot, and Tanjiro won’t be going anywhere soon. He’s slotted between his legs where his sworn enemy is laid out across the grime, looming over him, and though Muzan has all the cards Tanjiro still has this stupid defiant look on his face, blood running from him nose in an ugly line from when Muzan had landed his hit. He's lost his haori in the scuffle, only his uniform showing. The interesting thing- the thing that makes a wildly unfamiliar feeling rush his body, is that beyond the defiance in his eyes... is fear. He was alone, doing a pass of the city. No one is coming, no one knows he’s compromised, and he is completely at Muzan’s mercy.
He can see his chest rising and falling, breathing that's grown haggard with apprehension the longer Muzan leers. The more he stares the more his gut tightens and swoops, a heat pooling in his belly with Tanjiro spread out all small and helpless and scared, all of that caused by Muzan. Forbidden fruit he thinks, and he’s hungry. He dives forward before he can think, fisting the front of his uniform and hauling him up into a bruising kiss, teeth clacking together painfully.
The squirming mass of butterflies inside his guts is arousal.
Tanjiro makes a startled noise, too stunned by the action to struggle properly, eyebrows shooting up with shock. Muzan pulls away and chases the thrill of violence, using his leverage to slam the boy’s head against the pavement before kissing him again, against the alley floor. His uniform is streaked with dirt and grime, tarnished and dirty.
Tanjiro tries to turn his head away, choking out noises of protest that slip into Muzans mouth, but he chases his lips with painful force. He bites his lower lip hard enough to make Tanjiro gasp, sliding his tongue inside before he can recover and shoving it down his throat. He scratches at his chest through his clothes, hands roaming the smaller body with reckless abandon before he gives in and reaches down to palm himself through his pants. He earns himself a squeak of panic when he uses his other to paw at Tanjiro's groin roughly, feeling him start to harden against his will. Tanjiro recovers slightly, teeth scraping his tongue and Muzan grabs at him through his pants roughly, “No biting.” he snarls, giving a little tug as a warning, before he goes back to kissing him. He can taste blood on his lips and he knows it has to be Tanjiro’s.
When he pulls back a long strand of drool connects their lips, snapping as he leans away. Tanjiro is panting, wild panic in his eyes along with delicious fear, and this close he can hear his heart beating as adrenaline floods his system. Spit coats his mouth.
“W-what are you doing?” Tanjiro asks breathlessly, his shoulders straining against binds that won’t give, “Stop this.”
Ignoring him, Muzan settles his hands on Tanjiro thighs and squeezes lightly before trailing his hands up to grip the tie to his uniform, pulling it apart achingly slow. It’s like unwrapping a christmas present, peeling back the folds of his clothes like wrapping paper until all that's left is his undershirt. He rests both hands on Tanjiro's hips and stares hungrily at where his undershirt has just barely ridden up, a stripe of tan skin peeking out, and he inches his fingers up to slide just barely beneath the hem. Tanjiro shivers violently at the bare touch on his stomach, his breath quickening as he waits. Muzan wants to savor this, he wants to take his time, he wants to imprint the frightened look on Tanjiro’s face into his very being.
He slides his hands up up up , cloth bunching as he exposes more of his belly and chest, the smooth skin warm to the touch. He strokes the skin with bare hands, pressing along the curve of his muscles, perfectly sculpted and meticulously maintained. Hunger surges through him and he doubles down, licking a stripe from Tanjiro's belly button up to the valley between his pectorals, relishing the way his body shudders and jumps beneath him. He digs his fingers roughly into his ribs and he uses the sharp edges of his teeth to scrape along where his nipples were, and the rush of all consuming lust make his pants too tight. He keeps mouthing at his body, trailing his teeth up as far as he can with Tanjiro's shirt bunched around his neck, leaving scrapes all along the way. When he reaches the flat patch of skin just below his collarbone. he bites down as hard as he can, the sharp of his teeth leaving imprints in his skin.
Tanjiro jerks and it’s like the pain has reignited his brain as he begins to struggle in earnest, using his legs to try and scramble back but Muzan plants his palm on his tummy and presses him flat to the ground. Tanjiro whimpers as more pressure is put onto his shoulders, going nearly limp to entice Muzan to release him. He bites him again, so hard this time that he’s pretty sure he’s drawn blood and Tanjiro yelps, twisting at the sharp pain. Muzan slides his tongue along the indents of his teeth with such satisfaction it makes him lightheaded, kissing along the worst of the bites gently just to feel the marks against his lips. He bites him a few more times, before he sits back on his haunches to admire his work.
He groans at the sight, fuck, he could come from this alone, Tanjiro spread out beneath him pathetically, trembling like a leaf. His uniform is laid open, his undershirt rumpled around his collarbone to allow full view of the bitemarks littered across his chest, and Muzan shivers at how much they must hurt, how they would sting all exposed to the cool night air like this. He’s tenting in his own pants, cock straining almost painfully against the fabric, but he won’t finish here. He runs his hand along his well-toned chest, swiping at Tanjiro’s half-hard nipples.
Tanjiro whimpers at that, “No.” his voice shakes terribly as he twists his upper body to try and hide, “Stop touching me.”
He doesn’t even bother listening, too enthralled by Tanjiro’s perfect body. He moved a hand to roll one of his rosy buds, watching as it hardened at his touch. Tanjiro clamps his eyes shut, breathing through his nose as Muzan plays around at his chest. It’s morbid and it’s so fucking hot, and he wants to suck on them until Tanjiro cries. Instead, his hands jump down to Tanjiro's pants and begin fumbling to take them off.
Tanjiro shakes his head vehemently and protests loudly, kicking his legs out to keep Muzan from completing his task, “Stop kicking!” He hisses angrily, trying to grab at his legs and keep them still. He leans down to his ear and said, “Struggle, and I’ll make sure you will regret it.” he threatens, and while Tanjiro calms down for the moment he can see the ideas turning in his head, weighing his options, chest heaving.
He leans forward and rests his clawed hand against his thighs, the threat more visceral and serious, and savors the way he can hear and see Tanjiro's heart rate speed up with fear. He stops thrashing, holding impossibly still so Muzan doesn’t slip his hand, holding his breath. Muzan pats his upper thigh like he would a well-behaved dog, sliding one leg out of his pants for easy access, not bothering to take them off completely. He leaves his underwear on, admiring the cute black briefs and how they’re stretched across his half chub. A thin layer of cloth preventing him from his prize. He rubs along the length of Tanjiro's bare thigh, entertaining the thought of biting him again.
He and slides his sharp claws across the fabric to slice it away, his victim jerking at the action, the kiss of his nails too close to the skin of his dick. He feels Tanjiro's thighs squeeze around him, an automatic response to try and close his legs and hide away, “Don’t be shy.” he says, forcing his legs wider and Tanjiro bites his lip at the exposure, “You look good like this.”
It’s like this is where he was always meant to be, underneath the demon king, terrified and bare.
He slides his own pants down next, cock springing fully erect and he hisses at the cool air sliding over the sensitive organ. It’s been too long since he’s indulged in anything like this, not that he’s done anything other than jerk off alone, and the idea of dominating someone makes his head spin, especially considering who it was he was about to fuck. Tanjiro's half hard and Muzan reaches out, wrapping his fingers around it and giving an experimental pump.
Tanjiro's back arches and he cries out, looking up at him with wide wet eyes. Muzan leans over and presses their foreheads together, pumping at his cock rhythmically as he forces Tanjiro to make eye contact with him, “Has anyone ever touched you like this before?” Tanjiro bites down a sob as Muzan squeezes him lightly, “Am I your first?”
He doesn’t expect Tanjiro to answer, but the small, hesitant nod he gets shoots directly to his dick. Tanjiro is looking up at him through his eyelashes, eyes watering, and his lips are parted and wet as he pants helplessly through each stroke of his cock, and it’s so hot Muzan can’t wait any longer. He shoves two fingers into Tanjiro's mouth with no warning and Muzan groans as he gags on them, choking as he thrusts them in and out messily, coating his hand in drool. Tanjiro squeezes his eyes shut again as Muzan abuses his mouth, taking it silently- He wants to use his mouth, maybe break his jaw so he can open wide enough to take him all the way to the base, he’d look so pretty like that. He’d look so good sucking him off. (Next time, he thinks, his cock twitching at the idea of doing this again.) He blinks his eyes back open when Muzan pulls his hand free, his arms shaking with anticipation as he uses the spit as lube. Muzan can barely line his hands up, rubbing at his hole to coat it in his own spit. He sticks his pointer finger in with little anticipation, all the way to his second knuckle and Tanjiro yelps, his whole body tensing and trembling at the intrusion. He groans at the warmness around his finger and he presses on, working him too quickly and giving him no real time to adjust. He adds a second finger too soon and the boy whines, voice hitching painfully as he asks Muzan to stop once again, ignored like all the others. Muzan pumps and scissors his fingers, focused wholly on where he’s sliding into his body, roughly prepping him. His other hand kneads at his inner thigh absentmindedly, keeping his legs spread wide.
Good enough. He pulls his fingers out and shuffles forward, hiking his hips up and resting his dick on Tanjiro's body. Tanjiro is trembling all over, his shoulders straining under the force of how hard he’s trying to break out, and his eyes are wide and wet and panic-stricken. He’s so beautiful like this, face smothered in sweat and bitemarks along his chest. He spits in his palm and strokes it over his dick before he lifts Tanjiro’s hips up slightly, leaning forward and lining himself up and beginning to slide in.
Tanjiro chokes on a sob, and tears finally spill over and streak down his face. He closes his eyes and tries to hide his face in his shoulders, but he’s shaking and crying so hard he barely has the presence of mind to hide away effectively. Muzan is actually a little impressed he held out this long before starting up with the waterworks. He pushes himself in inch by inch, agonizingly slow. He wants Tanjiro to feel it, every bit of it, as he forces his way inside. He wants to split him open on his dick, spear him so deeply he can feel him through his stomach, and he wants to make this last. He sinks into the hilt, experimentally rolling his hips against the warm body he’s inside.
He cries so hard it sounds like it hurts, and Muzan reaches up to cup his face, forcing him to look up. Tanjiro can barely blink through the tears, heaving out each breath like they’re catching in his chest. He smooths his thumbs across his cheeks, sweeping away tears, “You’re doing so good, Baby.” Tanjiro's wheezes as confusion and shame and fear war on his face, Muzans sweet words contradicting so terribly with the violence that he doesn’t know what to think. The swirling emotions making it so much worse, Muzan waits until his sobs quiet down to shuddering gasps, legs trembling where they’re locked back in Muzans hands, and then he pulls nearly all the way out and slams back in full force.
Tanjiro screams as he spears him completely and Muzan leans over and kisses him, muffling his shouts as he begins to jackhammer into him, setting a punishing pace. He hopes each thrust is like a punch, he wants it to hurt. He wants it to be the only thing he can remember the next time he tries to fight him. He wants to mold Tanjiro's inside to his cock so no one else can ever compare. He sticks his tongue in his mouth, making out with him messily and swallowing each cry before breaking away and nipping at his jaw and his throat. He licks along the shell of his ears, trailing wet kisses along the sensitive parts of his skin before moving back down to his throat. He bites down on neck, leaving marks along his collarbone that he can see even in the low light. Tanjiro's' whole body rocks with each thrust, unable to find purchase enough to scramble away in the haze of pain and fear.
With his mouth free Tanjiro has dissolved into barely coherent babble, half baked begging all strung together as he tries to come up with some leverage to get Muzan to pull out completely. Muzan busies his fingers onto his chest, rolling and squeezing Tanjiro’s rosy buds, one hand holding Tanjiro's thigh to keep him in place. Fresh tears slide down his face and Muzan slides his tongue over his cheek, chasing the saltiness as he cries. He just keeps going, thrust after thrust, pushing as deep as he can, splitting him down the middle with each brutal thrust.
He’s getting close, and though he’s drawn it out so long, each snap of his hips brings him closer and closer to finishing. He moves his hands away from his chest, using his now free hand to grab at Tanjiro's own cock. The demon slayer writhes at the touch, shaking his head and distressed, begging him to stop, each word going directly to Muzans budding orgasm. He swirls his wrist, pumping out of sync with his thrusts and Tanjiro convulses at the stimulation, wanting desperately not to come.
He readjusts his grip on his thigh, grabbing at his ass and raising him up so he can get a better angle and hit even deeper. His penis brushes Tanjiro's prostate more firmly and Tanjiro wails, breaking into broken moans as Muzan aims for it over and over again. Muzan wants to be the best he’ll ever have, no one else comparing to his first time. Each pathetic keen and the shame and fear and pain on Tanjiro's face all compounds to send him over the edge, his thrusts becoming more erratic. He pumps Tanjiro's cock wildly and Tanjiro tenses and comes with a cut off cry, spilling across his own belly. He clamps down even harsher on Muzan and that's enough to bring him to his own climax, groaning as he finishes, pouring himself into Tanjiro's body recklessly. He thrusts through his own orgasm, riding it out and continuously jerking Tanjiro off, new tears rolling down his face as he overworks his sensitive dick to the point of pain. Finally he lets go, and he pulls out with a wet shlick , tucking himself back into his pants and sitting back.
Tanjiro is still trembling, aftershocks rocking his body after the sensation overload. He hiccups out these awful sounding sobs, trying to control his breathing but unable to calm down enough. “You look so fucking beautiful, baby.” he tells him breathlessly, tracing every part of him he can with his eyes, ensuring he’ll never forget this moment. Come drips out of him and Muzan tuts, using one finger to push it back in slightly before leaning down and biting one last mark into his inner thigh, and Tanjiro shudders and jolts at the contact, body sagging in exhaustion.
He leans forward, rolling Tanjiro onto his side so he can slice through the ropes binding him. New life surges through him the moment his hands are free and he scrambles away, pushing himself backwards until his spine hits a wall, curled up in a ball with his pants wrapped around one ankle. He locks his arms around his knees, keeping his legs clamped shut.
He looks up at him with confusion and fear in his eyes, Why let him go? Why not just fucking kill him already?
“What are you going to do, Tanjiro?” He says softly, staring at Tanjiro intensely, “Are You going to call your friends to come pick you up? Are you going to tell them what we did?” He leans forward, smiling an ugly smile, “Are you going to tell them how you came on my cock like such a little slut, baby?”
He’s shaking again, hands clenched together around his legs as if to keep them closed, his expression crumpling. Muzan knows he won’t say a word now. “Every time you try to fight me, you’re going to remember this. I’ve already ruined you.”
He stands and Tanjiro flinches back, the thrill that sends through him almost enough to convince him to try for round two. He licks his lips and turns away, walking out of the alley satisfied and satiated. He set out today to teach that boy a lesson.
He’s certain he succeeded.