Undyne watched, fascinated, as the human child removed their shorts and underwear.
Their cock was long and slender, a pale human cock with no foreskin and a plump pink head. The ten year old grabbed it and lifted it, first revealing a small ballsack just as smooth and hairless as the rest of them, then, when tugged up hard enough, a clit and the lips of a pussy. Instantly, magic pooled in Undyne's crotch as arousal came over her. The child surely couldn't be proposing this as a way to forge a peace, could they? Undyne had been told to kill all humans, no exceptions. That whole race was wicked and violent. What if this was a trick?
"You can use my holes," the small human said, "and then you'll feel better. We don't have to fight anymore."
Fucking a monster child was punishable by death, and for good reason. The psychological damage it did was incalculable. Most monsters would die from the anguish of the mental weight of it. Frisk, though, was human. Humans were more resilient. If Undyne ever wanted to go all out when screwing someone and not risk them coming to real harm, this was the opportunity. Plus, the human wasn't being forced. They were offering their wonderful body, with its' pussy and dick, up for use of their own free will. There was even a small glimmer of wetness inbetween their labia. This was a once in a lifetime opportunity. And Alphys had no cameras inside Undyne's house, though she had them outside. No one would ever know.
Undyne undid her pants. Magic pooled around her crotch, glowing, until it grew a full, thick cock for her to use on the human. The newly created appendage was made of flesh, hard and solid, conjured up just to enjoy this thoroughly. She picked the human up and put them on the table, spreading their legs. Kneeling, she gazed at that improbable anatomy, the asshole, the cunt, the small balls, and the dick. The head of the Royal Guard had never known such a human existed. So many dirty thoughts rushed through her head right away. Human children were small. Both holes were bound to be incredibly tight.
Standing again, she smacked her cock against Frisk's, making the child gasp. The cock Undyne had summoned for herself was three-fourths the length of Frisk's torso and nearly as thick as one of their thighs. Grinning, the monster ground her new dick against the human's childish cock, enjoying the apprehension in their face, the fear as Undyne used some cooking oil to lube up her fingers and begin fingering Frisk's pussy open. She would, she decided, try both holes out. First the pussy, then, when Frisk was good and compliant underneath them, the ass.
"You like having monster fingers in your cunt, kid?" she asked lowly, aroused by the little huffs of air Frisk gasped out as she worked two fingers into that narrow opening inbetween the human's legs. "You like it when you get put in your rightful place underneath monsters?"
Around her fingers, the walls of that small pussy tightened and loosened, squeezing Undyne's fingers experimentally. The child nodded, but remained silent. They were nervous, Undyne realized. She liked that. She liked having power over them. Rubbing her cock against theirs, she gazed down at them, at their cock's growing hardness and their pussy's increasing wetness. Oh, this was going to feel good. No monster had ever fucked a human before. At least, that's what she'd been led to believe. Asgore had ordered her to kill all humans. No human who passed through Waterfall had lived for Undyne... but none of them had offered up their bodies to her, either. This would be her secret, her fun, hers.
After lubing up her cock in preparation, she lined her tip up to Frisk's slit. Watching the child's face carefully, she grinned, then thrust herself abruptly into that wet and welcoming hole, moaning as its walls tightened around her. Without waiting for the human to adjust, Undyne forced the entire cock down into their cunt. Frisk's face was exquisite, a portrait of pain and discomfort, even a little fear. She pumped in and out of them, immediately going for brutal pace, the small and flexible body offering up no resistance. They were trapped, under her complete control, at her mercy.
"Ah... ah... ah..." the ten year old panted, with each fuck of the monster's cock into their pussy. Their stomach bulged with every stroke into them, distended by the massive organ inside them. "Oh, oh... oh...."
"That's it," Undyne groaned, "take a monster's cock in your tight cunt, take it and get fucked, kid. Yeah, spread your legs for me, let me get in there and pound that sweet wet human pussy."
Frisk obediently spread their legs, putting a hand to their mouth and biting onto the sleeve of their sweater for support as the rough, fast pace continued. Their cock was erect now. It shifted with the thrusts, its head rubbing on Frisk's abdomen. They whimpered but didn't dare move to touch themselves. Their free hand was gripping the table's edge for support, although the table itself was rocking with the force of how hard they were being fucked. Despite the pain of the intrusion into their insides, the child was erect and wet, their cunt beginning to pulsate and clench around the enormous cock shoved deep inside it, and as they met the captain's eyes, a blush and timidly pleased expression crossed over their features.
"Little human fucktoy. Little human cocksleeve," Undyne groaned. "Love the sound of that. You belong underneath a monster, servicing a big monster cock. Your pussy was made to take a great, big dick in it."
The monster rutted furiously into their tiny body, enjoying the give and take of their youthful body as their belly stretched and contracted, in time with the penetration into their pussy. There was no escaping the fact that a dick this big inside them hurt. It ached. Their pussy was being stretched and warped by Undyne's cock, and there were no safety mechanisms that could reverse or stop the transformation occurring inside the child, the forced relaxation of their cunt's walls, the constant battering and pushing up against their g-spot. Moaning and whimpering, the human tried to shift their hips, to lessen the pain of it, only to have Undyne pin them down on the table. Her eye glinted with magic. The ten year old swallowed nervously. Their pussy was squeezed with what felt like immense pressure as Undyne kept working herself all the way into their tight tunnel.
"Why do you keep moaning and whimpering, kid? You want my dick that badly, huh? You some kind of fucktoy on the service? Everybody uses your holes to get off on? I bet that's it, I bet you're a cocksleeve for other people and they fuck this pussy all the time," the large teased, whispering the dirty words. Her cock pushed ever deeper in, breaking through the membrane guarding their cervix. Undyne grinned, hearing the shriek of pain that Frisk let out, and let the tip of her dick work itself through their tight little opening into their womb. "All these fuckers have probably been using you, that's it, stretching your slutty pussy and putting a load of come in your hole, stretching out your hole and forcing you open for their big cocks."
Frisk gripped the table with both hands as Undyne kept pounding them. The penetration of their cervix was pure pain. It was agony, pure and simple, making them groan and pant in distress, trying to contain their moans. Undyne's teasing words hurt. Their dad fucked them, yes, but only him. Only their wonderful father used their holes, and he had been so much gentler, so much better. This felt like getting stabbed inbetween the legs. The human tried to remember how to stay calm even as the monster woman's huge cock was shoved again into their deepest parts, forcing its tip against their sensitive wall and grinding.
"Take my cock," she demanded lowly, "take my fucking cock, take it and say thank you! Thank me for not killing you!"
"Th-thank you!" the child said, shaking as Undyne rammed it as deep in as it would go and held it like that, their stomach bulging, so full they genuinely feared for their safety. "Thank you for n-not killing me!"
The captain of the Royal Guard pulled her cock out of Frisk's pussy, leaving it gaping. Any relief Frisk might have felt vanished as the monster began rubbing the tip of her dick into that tight asshole, grinning. Her teeth, jagged and sharp, gleamed at the child. Panic shot through them. They'd never had anal sex before with anyone other than their father, who was much, much smaller than Undyne. Frisk had hoped, when they said she could use their holes, that she would only use one and then let the human go. Instead, the head of her cock nudged insistently at an entrance so small and unstretched it couldn't yet be breached.
"What, you think I won't wreck your asshole too?" Undyne glowered down at them. "This is where you belong. After everything humans have done to us, this is what you deserve." She grabbed Frisk and told them, "wrap your arms around my neck."
Frisk did as they were told. Undyne lifted them off the table, holding them to her body as she lowered them onto her cock. Aided by gravity, the huge length was pushed in. The human screamed in pain. Their voice hit a volume and pitch they'd never known they could. The cock split them, forced open a passage that was not big enough or accommodating enough, no amount of wetness or friction able to soften the girth invading their anus, going up and in, towards their belly and their lungs and their heart. Frisk's body spasmed helplessly around the monster's length, impaled.
Undyne began bouncing the human on her dick, moaning. "Oh, fuck, you're such a hot cocksleeve. Your body is like an inferno around my dick. Oh, yeah, mmm," she gasped out in delight as she raised the human's body up and down atop her cock. "Take my cock, take my fucking cock!"
"Hurts..." the child murmured as they cried. Their tightest hole was ripped open, the painful fullness in their gut driving them to distraction. Tears slid down their cheeks, making a salty trail to their chest.
The monster merely moaned at that and continued happily bouncing them on her cock. She had never felt anything this tight, this hot. Fucking a child was a new kind of pleasure, one she would never forget. And the human had no choice but to take it, body jostled up and down, serving as a sleeve for the captain's dick. She was lost in how good it felt. She had dominated the human, proven her strength as she fucked them, proven that if anyone else who was human dared attack her, this would be what they had to look forward to. Monster cock wrecking their holes, stretching them out and wrecking their insides forever.
She began moving her fucktoy up and down faster as she thrust into them again and again. Undyne could feel the slickness of organs against her dick. Moaning, the monster watched the human's face intently as she ruined their body. Her cock went up into a new, warm chamber as the human began spasming again, their entire body contracting against her and convulsing as they came. With a scream, the child arched their back as their insides contracted around Undyne and a torrent of liquid squirted from their pussy. Their pussy, she noted, but not their dick, which was still hard. Not only was the human getting off on being fucked apart, they were still horny. The realization made Undyne feel even more powerful.
"Yeah, oh yeah, squirt on my cock," she breathed out, letting their entire body drop back onto her length, taking her to the hilt. "You love being fucked on monster cock, you slut, you cocksleeve. You want more. You want to be fucked until your little cock comes just like your pussy."
Frisk's gaze grew distant for a moment from the sheer pain of having been hilted anally on a monster dick. They could feel their organs shift inside them, their belly distended hugely by the inhuman penis forced into them. The human's body spasmed so badly only Undyne's arms around them kept them from collapsing, their tiny balls drawn up as far into their groin as they could possibly go, their cunt tensing repeatedly. Everything felt wrong, every movement made the length in their rectum scrape them inside, made their prostate receive direct contact and pleasure them unrelentingly. Their insides were on fire. They were dying. The pain kept hitting new highs. And their dick stayed hard despite it all. Their pussy gushed in a way it never had before in their life, as if it were trying to force a climax the child just couldn't reach.
Their body continued trying to squirt as their asshole tightened around the intruding flesh. After coming and failing to completely squeeze the orgasm from themselves, their muscles seized. They panted and cried from the exertion, body overtaxed by what Undyne was doing to them. But the woman didn't stop. She renewed her movements, bouncing the ten year old forcefully on her cock, smirking as she forced it all the way back into their ass each time.
"Yea-aaah," Undyne drew the word out, elongating the vowel. "That's it. Good kid. Good slut. Be my human bitch. Let me take this hole apart, too."
The cock plunging into their asshole was brutal and forceful. Frisk had never felt so raw, so violated, so utterly useless and powerless to resist. They were ashamed of their cock's continued hardness. Though pain was overtaking their senses, they remained turned on. It was horrifying to feel like this, like they were an object for Undyne to use to get off with. As Undyne picked up the pace, the child screamed. Blood and lubrication oozed from their ravaged hole. The pain only made them tighter.
The child arched against the woman, whining desperately. They moaned and cried at the same time. Undyne make lewd groaning sounds as the room echoed with the slap of skin on skin, wailing and dripping of fluids. With their cervix destroyed and their insides fully fucked open, Frisk was very wet, the thrust of Undyne's dick causing copious squirting with each move into their body. Even their cock spurted precum, although cumming seemed like an impossibility. Blood was dripping out of their ass. Still, despite their wails of agony, the fucking continued.
"Your asshole feels good around my cock!" Undyne screamed, fucking Frisk until their entire body, loose with exhaustion, could do nothing except let themselves be bounced along the huge organ, their throat raspy with screaming, their pelvis broken, smashed apart by a cock and ripped open. "Take my cock! Take! My! COCK!"
With the monster woman's final words, she moaned. Frisk shrieked, legs shaking, as their insides were set aflame by semen. It was scalding hot against their internal organs, and worse, there was so, so much it! They went into a seizure, body filling up in place never designed to be filled. The heat washed over them just as much as the pain, making them feel like they were burning up. Somehow, Undyne kept shooting cum into the child's body, lodged as deep in their tiny body as she could be. The woman moaned as she emptied it out into the ten year old, leaving her fillings dribbling back out with every few moments of orgasm.
"This is it, kid, this is my load for you," she hissed in satisfaction, loving the power and control she had. "Now come for me. Come for a monster, on a monster cock, with monster cum inside you!"
When Undyne grabbed Frisk's shoulders and shoved them down harder onto her massive cock, Frisk screamed so loud they felt their throat give out. Their traitorous cock, though, finally grew overloaded, and a wave of vile pleasure rolled over them as they came. Their cum spurted across Undyne's tunic and breastplate as she fucked the little ten year old to a shameful orgasm, forcing it out of their tiny body. They squirted, too, but that only added to the sick thrill of making a child orgasm. Frisk's screams were cut off abruptly from exhaustion and the overstimulation, and as their eyes shut, Undyne thrust deeper and continued pumping her cum out into their ravished body. The last thing Frisk remembered before fainting was the Captain saying, "what a human deserves," before the heat, fullness, and stickiness became too much and consciousness left them.