[Kimetsu no Yaiba] the disappointment

Title: the disappointment
Fandom: Kimetsu no Yaiba / Demon Slayer
Pairing: Rengoku Shinjurou / Rengoku Senjurou
Words: 2,354
Completed: 2023/06/08
Rating: R18; For Adults only!!
Trigger Warnings: Incest (Father/Son), Non-Con, Underage, Omegaverse (breeding, marking)
Comments: This is my first attempt at smut and non-con. Please mind the tags. Don’t like it, don’t read.
Summary: “That’s it, Senjurou. We’ll continue the line of strong and healthy warriors. Just you and me.”

Senjurou was a disappointment to the Rengoku name, but maybe his father still has some use for him after all…

Links: AO3 & FF.net

Rengoku Senjurou was a disappointment.

That in itself was no surprise, least of all to the young Rengoku himself, given his inability to keep up with his father’s training regime in his early years, and even later, when his older brother took over and tried to train him in his father’s place, Senjurou fared no better. Frankly, it was no wonder that his Nichirin Blade never changed its color and thus he was the first Rengoku in a long line of generations not to become a member of the Demon Slayer Corps, robbing the boy of his only chance to ever be able to wear his family’s pride, the flame haori, just as his Aniue once had.

In the end, the line of fine warriors in the Rengoku family ended with Kyoujurou’s death.



Rengoku Senjurou continued to prove to be a disappointment, though.

“No son of mine is an Omega!!” 

The now-last Alpha of the House Rengoku roared and slammed his jug of sake against the wall, staining it with sake. Unimpressed by this outburst, the doctor repeated his diagnosis again in a monotone and matter-of-fact voice: “Rengoku Senjurou is an Omega. His uterus is already fully developed and he will be able to conceive children during his first heat, perhaps even sooner.” He looked Senjurou in the eye, who stiffened under the doctor’s gaze, “He’s quite the fertile one, so a pregnancy outside of a heat shouldn’t come as a surprise.”

For others, this might have been the highest praise one could only imagine receiving, but not for a Rengoku. In their long family history, which dated back many centuries, there had never been a male Rengoku heir who was an Omega. Not a single one. And now Shinjurou’s last son turned out to be an Omega?!


“OUT!” Shinjurou roared, standing imposingly in front of the physician. “Out, now!” He belled once more for good measure. It might have been an open secret that the eldest Rengoku was nowadays more bark than bite, but he was still a former Hashira, magnificent in size and with an overwhelming presence. The doctor did not need to be told twice and excused himself as he hurriedly left the Rengoku estate.




How did it come to this? 

One minute Senjurou was sitting in the common room watching the frightened doctor practically flee their house, and the next moment Shinjurou grabbed his son by the wrist and dragged him into his bedroom. The clothes still sat very loosely on the boy’s body – he had never had a chance to dress properly after the examination – and Senjurou tried his best to keep them on. His efforts were in vain, however, as the older man tore off the boy’s clothes and pushed the completely exposed Senjurou onto his futon.


Senjurou was scared. Yes, his father was an easy-to-provoke man and it didn’t take much to bring him to a boiling point, but this… this was different. Shinjurou wasn’t just upset, no, he was seething with rage, something Senjurou had never seen in his father before.

And that rage was directed at him.

“On your stomach.”

The commanding voice cut through the air and Senjurou shivered, fear filling his heart. 

This… this wasn’t his father’s voice.

This was…

“On your stomach, I said!”

Senjurou’s vision turned upside down and the boy found himself face down on the futon, both of his wrists captured skillfully with one hand and pinned against his back.

What was happening??

“My own son…”


“A fucking Omega…”


“What a disgrace.”


Senjurou willed himself to keep his mouth closed, lips pressed tightly into a thin line. It had been years since his father had last raised his hand in such a manner, but the memories from the past were still freshly burned into the boy’s mind. Every scream, every whimper would only provoke the older man further and made the next blow all the more painful.


When silence fell after the last hit, Senjurou allowed himself to breathe a sigh of relief. Maybe he was lucky this time, maybe this was all-


Senjurou’s scream pierced the room and tears shot into his eyes. 


“Fa-father, please. S-stop!”

Father, why are you doing this?



However, his cries and pleas went unanswered as Shinjurou’s finger continued to explore Senjurou’s dirty little hole. If anything, they further fed the wicked grimace on his face. Might as he tried, Senjurou was unable to squirm free as his strength simply was no match for that of the former Hashira.

Father had always been stronger.

The boy’s screams filled the room, begging the man to ‘please, please don’t!’ until his throat ran dry and started to hurt.

He was the most intimidating man Senjurou knew.

Cold tears streamed down his face and Senjurou tasted their salt in-between sobs.

It was child’s play for Shinjurou to overpower his youngest son at any given moment if he wished so.

“S-stop it… p-please F-faAH—”

The older man clicked his tongue clearly dissatisfied, and with an abrupt motion freed Senjurou’s backside from its torture, making the boy shudder at the vulgar sound it made.

“Tch, can’t even get wet by yourself.”

Against his better judgment, Senjurou hoped his father had had enough and would let him go, but the boy was caught off guard when the touch returned to his most intimate body part, this time more slippery and moist than before.

No, he shuddered and an uneasy feeling rose in him.

No, he can’t—!!

“Hrng…” The spit hardly eased his discomfort and Senjurou hissed as each friction was accompanied by a searing, burning pain. Undeterred by his discomfort, Shinjurou continued with a steady pace, adding more saliva whenever needed for his own pleasure. Senjurou continued to squirm and struggle underneath him, crying and begging all the same and the Alpha huffed an irritated sigh.

“Even as an Omega you’re a failure, huh?” He scoffed while adding a second finger, stretching Senjurou’s hole further than before. The boy yelped at the unwelcome sensation and his shoulders shook in rhythm with his sobs.

“Raise your hips.” 

The commanding tone of the Alpha left no room for disobedience, and on shaky knees Senjurou did as he was told, wincing halfway through. A shaky breath escaped him and his whole body vibrated, a tingling sensation shooting from his butt to his head, leaving the boy short of breath.


Senjurou froze, startled by the sound that had just passed his lips. In the back of his mind, he heard his father humming gleefully, and a shiver ran through his body as yet another groan escaped him.

That sound just now.

What… what was that?

Did he make it?

No, it couldn’t be. He didn’t sound like that – Senjurou would know! – and yet…


And yet, there it was again. Stunned, the boy realized that his cock was twitching uncomfortably beneath him, and he bit his lips to stifle his voice.

This couldn’t be happening.

“Aaahh… Father….”

The pressure on his back that held his wrists together was gone – since when the boy could not say – and Senjurou propped himself up on his elbows. His cock, free at last, stood in its full glory, throbbing, and the first drops of precum gathered on its tip. A third finger penetrated him and more and more moans escaped his mouth. He continued to beg, just as he had done before, but this time he sang in a different tone.


A feeling Senjurou has yet to name or even acknowledge flared up inside of him, slowly chipping away at his senses.


To resist was a futile attempt right from the beginning.

“Please… more…”

It was Shinjurou’s hollow laughter that shook Senjurou’s core and brought the boy back to reality, almost as if someone had poured ice-cold water over him, and shame bloomed in his chest. Senjurou was ashamed of his own body for reacting the way it did, ashamed that he felt pleasure under his father’s touch, but most of all, Senjurou was ashamed that he had begged for more.

That he was still begging for more.

“Are you finally showing your true colors?” Snarled Shinjurou, a grotesque laugh hiding in his voice. Senjurou’s breathing hitched, fingers penetrating him deeper than before, seemingly in search of a specific spot and leaving the boy boneless as they suddenly thrust.

Aahhhh!” Senjurou’s high-pitched scream echoed through the room, making even his own ears ring, and for a second or two, his vision went black, leaving the boy gasping for air. His whole body shook from the aftershocks and his mind went numb to his surroundings.

What was that??

His body was crying out of pleasure. It was so much (not enough), too much (more, he wants more).

“Ah, there it is.” Shinjurou’s voice carried a sinister tone and his fingers rubbed the sweet spot again with renewed vigor, sending jolts of pleasure through the child’s body. Senjurou was barely able to maintain consciousness, moaning one scream after another and quickly beginning to chase after more.

He grew ecstatic under Shinjurou’s skillful fingers, but with every new moan, he was also growing more and more impatient.

Not enough.

The boy raised his hips as high as possible, moving them somewhat clumsily – but with enough enthusiasm to make up for it – as he tried to match his rhythm to Shinjurou’s.

“Oh ho?” Shinjurou paused for a moment, watching in glee as his son moved his hips up and down, sometimes in circular motions, but gradually increasing his pace.

“Who knew I have a little whore for a son, do you enjoy being fucked by your father’s fingers that much?”

Senjurou could barely understand the words being spoken to him as he worked himself into euphoria on his father’s fingers, eyes gleaming and mouth open as he gazed at something far, far away, a delicate trail of drool leaving his mouth.

His father, however, had other plans for the boy. With a plop, he released his hand from Senjurou’s gaping hole, and the boy immediately whimpered, not at all pleased with the emptiness inside him. Droplets of love juice leaked from his hole while his cock was covered in precum.

“Look at you, leaking from every hole. What a sight you are.”

The boy whined, thirsting to be touched again and pheromones leaked out of his body, calling for the Alpha.


Calling to be taken.


Shinjurou loosened his obi and discarded his fundoshi, which was already stained and translucent with his own precum. Senjurou felt his father’s hot and throbbing cock pressing against his entrance, and he shuddered with fear and anticipation.

“Hng, you are so tight.”

Senjurou’s cry was cut short as Shinjurou pushed into his hole with a powerful thrust, filling the boy to the hilt with his cock and almost tearing him apart. Shinjurou’s fingers had not prepared him enough for the massive cock inside of him.


Without listening to his son, Shinjurou began to move, slowly at first, but then faster and faster. With each thrust, stars rose in Senjurou’s eyes, and a feeling of pinpricks ran through his entire body.


Senjurou could no longer speak, his mind clouded with pleasure and the burning desire for more. The slapping of flesh combined with the boy’s moans and the occasional grunt filled the room, all in the same constant rhythm that put Senjurou into a trance.

Senjurou was sure he was on fire.

The air was thick with the Omega’s pheromones, and for the first time since he began his assault Shinjurou’s breath caught. However, his astonished expression was soon replaced by a wicked grin. He growled as his Alpha senses took hold of him, burying himself deeper into his son as he quickened his pace and made sure each thrust hit with full force.

“Can you feel it? Your womb opening up, ready to receive my seed?”

“Hnnnggghh.” Shockwave after shockwave coursed through Senjurou’s overstimulated body and everything inside him was on fire. He was burning and chasing and getting closer and closer. The walls around Shinjurou’s shaft tightened and squeezed him, eagerly waiting for the precious seed to be released.

“You want my children so bad, huh? Already trying to squeeze out every last drop of mine before your hole is even filled.”

Senjurou nodded frantically, chanting ‘Yes, yes, yes’ and ‘Babies. I want your Babies’ with every moan. His mind was reduced to pure, animalistic pleasure, only able to move his hips in rhythm with his Father, always chasing, chasing, chasing for the sweet release and to be filled with the precious seed.

More. He wanted more, more, more. 

Alpha. Alpha, Alpha.

“Carry my children.”

The command paralyzed Senjurou and his mouth hung open, a silent scream passing his lips, the duvet long since stained and soaked with his cum leaking from his little cock. His breath hiccupped and he shuddered at the sensation of blood dripping from his neck, leaving a tingling trail. It had to be painful, Senjurou was sure, because his father was not known for holding back and proudly licked the blood from his lips, but all the Omega was able to feel at that moment was pure ecstasy.

My Alpha.

“You are mine now.” 

Yours, yours, yours.

“Hngh.” Senjurou tensed as his hole stretched wider and wider. Shinjurou’s knot, massive in size and volume, continued to pump his seed into Senjurou’s tiny womb, filling it to the brim and beyond until it overflowed and sperm spilled from his hole, tainting the futon with great puddles of thick, white semen.

“That’s it, Senjurou. We’ll continue the line of strong and healthy warriors. Just you and me.”

As his belly swelled from all the semen it had been filled with, Senjurou couldn’t help but feel strangely calm. Blissful even. It was no more than an afterthought, but for the first time since he was born, the young Omega felt content. He felt useful.

At last, he had found his purpose in life.

“Yes, Father.”

Rengoku Senjurou would never be a disappointment again.


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