Prompt: After watching Gilgamesh effortlessly defeat Archer and Berserker, Caster TOHSAKA is forced to acknowledge that she has no hope of winning against him. But, they still have a hope- her Master, Shirou Emiya has a Reality Marble perfectly suited to opposing the King of Heroes. There is just one problem, he doesn't have the od output needed to keep up with Gilgamesh. The solution is obvious... transferring the Tohsaka Magic Crest to Shirou to allow him to match the King of Heroes and more. Rin was the first volunteer, but Caster was also a Tohsaka...

Tags: Shirou Emiya/TOHSAKA Rin, femdom to maledom, sex, I have no idea if this counts as NTR


The Counter Guardian TOHSAKA in this chapter uses the design of the Formal Craft CE (Basically FSN Rin with a lot more generous curves) as per the prompter's request.

Link for reference image: https/fategrandorder./wiki/Formal_Craft


The situation was less than ideal. Gilgamesh had the Holy Grail- having murdered Illya directly in front of them. Shirou had been helpless to save her. Then they had attacked Jason and his Argonauts with the help of Lancer. The arrogant bastard who dared to flirt with his servant. They had won, but Archer had betrayed them. He had kidnapped Rin and told Shirou to come to the Einzbern castle to fight him. Shirou had obeyed. Caster had tried to protest, but Shirou had to face Archer himself. Caster had watched, ready to heal him if he wanted out, and clear an exit for them. Shirou had of course refused all of her aid.

Shirou Emiya had won against the perversion of his ideal. Or at least he was going to win. He had finally managed to strike back, seizing control of the Reality Marble, and landed a blow on Archer's torso. Because Archer had held back. He was going to beat reason into his future self, make him see that their ideal had value.

Their battle had been interrupted by the King of Heroes. The murderer of Illyasviel von Einzbern. The Archer of the Fourth Holy Grail War, King Gilgamesh. He had snapped his fingers and a dozen Noble Phantasm had flown out of a portal and skewered the Archer, injured and tired from his fight with Shirou.

Gilgamesh had the Holy Grail. They would have to defeat him to end the Grail War. And they would have to destroy the Grail to end this. It was an impossible task.

"I will try to fight him, of course, but I will lose," Caster said lazily. She leaned across the dinner table, their food finished hours ago. She looked like she wanted nothing more than to not be here. "Goldie has no doubt used his abundant riches to gather all of the magic resistance in the world. I doubt I'll be able to do much more than scratch him."

"What about your Noble Phantasm?" Rin demanded. "Does that have any chance of evening the playing field?"

Caster scoffed. "Please, you just want to be spoiled on what will be the greatest achievement of Rin Tohsaka." She let out a weary sigh. "I suppose I have no reason to hide it from either of you at this point. My Noble Phantasm is the Jeweled Sword of Zelretch."

"I'm a Magician!" Rin gasped.

Caster shook her head sadly. "Just his apprentice, and the Jeweled Sword, while powerful, causes my muscles to atrophy from repeated use. And it's less effective as a defense against someone throwing swords. In order to damage him if we assume his magic resistance is of the highest grade, I would have to self-destruct the sword. I could maybe get lucky, and kill him that way, but it's a one in a million shot."

Rin groaned. "Why am I so useless?" she muttered to herself.

"I could fight Gilgamesh…" Shirou volunteered hesitantly. It was not that Shirou was not willing to fight Gilgamesh. It was that both the Rin Tohsaka of the future and the present had spent the last two weeks trying to beat his need to do it all himself out of him. Everytime he volunteered for anything he was scolded for it. And Archer had been insistent that they didn't save people from a desire to see them saved, but a desire to do it himself, a borrowed ideal. He didn't think he was wrong, but it was what caused him to hesitate.

"Hmmm." The two Tohsaka's hummed in sync, as they considered the idea.

Shirou felt obligated to say something more to their identical, judging stares. "Archer… As he died… he told me that Gilgamesh is no match for us. That we would fare better than Rin. He said that I should defeat him."

"My servant talked to you when he was dying instead of me!" Rin huffed.

"I talked to him too!" Caster chimed in cheerfully. She had just told him one thing but…

"Shut. Up."

Shirou tried to bring the conversation back to what was important, before the two started screaming or god forbid, fighting again. "I can fight Gilgamesh."

"He has a point-" Caster interjected.

"Of course, he has a point!" Rin snapped. "Gilgamesh obviously saw Archer as a threat. Their magecraft is the perfect counter to Gilgamesh."

"It is?" Shirou gasped.

"You didn't realize it?" Rin gaped at him.

Caster took over from there. "What makes Gilgamesh so strong is the number of Noble Phantasms he has. But to put it another way, one-"

"I was going to say that!" Rin shrieked. "Stop interrupting me!"

"You should've-"

"Tohsaka," Shirou warned. Both identical eyes swerved to look at him. He could feel another headache approaching. He could not imagine surviving without her and he did genuinely care for Caster, but did he have to summon another Rin Tohsaka? One Rin was already far too much for any man.

Caster sighed and let Rin resume her explanation. "What makes Gilgamesh so strong is the number of Noble Phantasms he has. But to put it another way, one can match him if one has the same number of Noble Phantasms."

Shirou refrained from pointing out that she had said the exact same sentence, word for word, that her future self had tried to say. Sometimes their resemblance was a bit too uncanny. "Having the same number of Noble Phantasms might allow one to match him, but I'm not Archer, I can barely project two Noble Phantasms!." And that was the problem. Archer was a match for the King of Heroes, but he had died protecting Shirou from his blades. And Shirou lacked the magical energy that Archer had as an agent of Alaya.

"That's if your magical energy is just like what it was until now," Caster explained.

"Let. Me. Speak." Rin growled at Shirou's servant. In a very un-Rin gesture, Caster stuck her tongue out at her younger self.

"Archer's Noble Phantasm was his Reality Marble," despite the fact that Rin was addressing Shirou, her eyes didn't stray from Caster, glaring at her fiercely. "You can match Gilgamesh if you use it." She finally pulls her eyes away from Caster to look at Shirou, her eyes filled with hope.

Shirou had to dash those hopes. "That's impossible. The Reality Marble is the ultimate forbidden magecraft. I don't know how to use it, and Archer used magical energy several times my capacity to create that world."

"Yeah, it's impossible," Rin was quick to agree. "But you should know how to use it- all of your magecraft, both strengthening and projection stem from it. Once you have the necessary magical energy I'm sure it will be a piece of cake."

"Caster?" Shirou asked the more experienced Rin Tohsaka, trusting that she would know more about this, as she did about most things- a lifetime of experience tended to do that. Rin stomped her foot in frustration.

Caster smiled at Shirou, her entire face lighting up as she flashed her pearly him. "I have only ever seen one Reality Marble, but yes, it should work that way. Reality Marbles are not like most magecraft. They function on a more instinctive level, centered more around the mind and the 'Alien Common Sense'. They don't rely on rote memorization or repetition."

Rin refused to admit that she found Caster's explanation better than her own. Whatever, it was her who was saying it anyway! She still got the credit!

"...Okay? But even with Caster's tutelage over the last two weeks, I still don't have the magical energy necessary to set up the bounded field, let alone maintain it."

"I know… As Gilgamesh said, when a mage can't provide their own magical energy, they take it from someone else," Rin explained before Caster could say anything.

"Like Caster's gems?"

Rin grit her teeth at him calling them Caster's gems. She had gems too! They were hers! And Caster was her! Just say like your gems , that dumb stupid idiot! "No. Only I can absorb energy from my gems."

Rin bit her lip. "There are a lot of procedures in order to use someone else's magical energy as your own, but the only thing we can do right now is that one, so…" She trailed off, unable to say it out loud.

"Rin?" Shirou prodded. "Mind letting me in on what your idea is?"

Rin watched as Caster opened her mouth and Rin beat her to the punch with a snarl. "It's a magic circuit transplant, okay!"

"By magic circuits you mean…? Are you really willing to do that? We can find another way."

"We can't," Caster cut him off cheerfully. "The only way to make sure your Reality Marble is able to defeat Gilgamesh is to give you new Magic Circuits."

"Right… So I'll… transfer some of my circuits to you. Caster will facilitate the transfer… I think the safest move is to transfer…" Rin tried to explain, but her voice was unsteady with apprehension. Both to losing her circuits and what it entailed.

"You'll do nothing, little me," The voice of Caster cut off her ramblings. "I will be the one to transfer my circuits to Shirou."

"What! That's ridiculous!" Rin protested. "I already volunteered-"

"The ten circuits you intend to offer won't be enough. Shirou's body functions around 27 circuits, as broken as they are. Inserting ten at a higher level without stabilizing them will leave him unbalanced and while his output will improve, it won't be enough to maintain the Reality Marble for long enough to defeat Gilgamesh. Nor will it-"

"I wasn't going to offer ten!" Rin insisted, a lie . "You didn't let me finish! Besides, a Heroic Spirit can't transfer circuits to a human! His body would reject them! The sheer magical energy output is too high for his body to contain! And your spirit origins aren't close enough! He will fall apart if he tries to absorb your circuits!"

Caster smirked at Rin as she leaned back in her chair. Rin wanted to punch herself. "That would be true under normal circumstances, but that is why I will first transfer my crest to him. Only after will I transfer all 40 of my circuits to Shirou. The crest exists in a different plane from the body already, and as you know, is naturally synced to the circuits of a Tohsaka. It will allow him to adapt to all 70 of my circuits. He will have more than enough to defeat Gilgamesh, and to deal with what comes afterwards."

"You can't know that!" Rin argued vehemently. "It's too risky! I won't let you gamble Shirou's life on a maybe!"

"It's not a maybe. The odds of him surviving with 97 magic circuits after the transplants are higher than the odds of him defeating Gilgamesh with just your ten. I am willing to stake all of our lives on this assertion."

"I'm not! Maybe ten circuits aren't enough, but I can offer my crest instead! Thirty circuits should be more than enough to defeat Gilgamesh!" Was Rin really arguing that she should be allowed to forsake her family legacy? Why was she doing this? She looked at Shirou, and her cheeks flushed. Right that.

"Thirty is still less than seventy. Would you like to never use magecraft again?" Caster's eyes kept on drifting over to Shirou, a smile on her lips. Her leg was bouncing up and down under the table. How could Rin have ever grown into that much of a whore? "After all that our father sacrificed?"

Rin winced at the low blow. "Why are you so desperate to do this, anyways?" Rin snarled, she was being a hypocrite. "Why do you want to lose all your circuits? You would be powerless without them! You would be able to do nothing about Gilgamesh or the grail! What is your angle here?"

Caster shrugged, her eyes never leaving Shirou's broad chest, the smile never leaving her face. "I am a Counter Guardian. I already told you that. I might not be as desperate as Archer, but I have no desire to serve Humanity for a minute longer. Losing my circuits makes me useless to Alaya, and would free me from service."

"That is just a hypothetical," Rin argued. "A completely ridiculous one that is highly unlikely to succeed."

Caster finally glanced away from Shirou to take in Rin. She smiled at her patronizingly. "Probably."

"This is stupid!"

Caster turned back to Shirou, and the hussy thrust her annoyingly large chest out towards him. "The choice is your Shirou. Do you want to have me and my seventy circuits of untold might? Or do you want the little version, and to ruin her prestige and lineage forever for a lesser reward?"

Rin knew it was an obvious choice. "Caster might die… If it works and she loses all of her magic circuits… Her body could break apart and dissolve, her soul lost forever." It was a true statement, not just her crying for him to pick her. Even if that was a large part of why she said it.

"Highly unlikely."

Shirou took them both in, his gaze flitting from one to the other in quick succession. "Archer… he would've done anything to… I don't want Caster to become like that. If she wants to… I shouldn't stop her… I'd prefer she doesn't die… but if she does… so be it."

"But-"

"I trust Caster. And I don't want you to lose your crest Rin, that is important to you, and your family. I will do the transplant with Caster."

Rin rose from her chair with speed that surprised Shirou and caused Caster to grin. She stomped her foot and glared at her future self. "I'll go prepare some things for the ritual then! I'll be back in an hour!" She stormed off to her room in a huff, the door loudly slamming behind her.

"Is Rin okay?" Shirou asked Caster after hearing the door slam. "I thought she would be happy about not losing her magic crest?"

Caster smiled at him warmly, her eyes alight with joy. "We had prepared ourselves for this… she really had her heart set on having sex with you."

Shirou's eyes bugged out of his head. "What!? S-Sex!? We're doing what?"

Caster recoiled, her hands nervously folding over themselves. "... You didn't know… This whole time… I thought…" She muttered quietly, her earlier confidence shaken. In spite of herself, Caster couldn't help but ask: "If- If you knew, would that have changed your choice…?"

Shirou was still a bit shaken, but was quick to shake his head. "I still would've chosen you Caster, if I have a chance to save you, then I am going to take it."

Caster sighed in relief and then beamed at him. "I'm glad. You have no idea how much that means to me." An awkward silence filled the room, neither of them sure how to proceed from here.

"So uh, lovely weather?" Caster tried to make small talk to fill the awkward silence that had descended upon the room.

Shirou nodded, his eyes not leaving her for a second. She had always been a beautiful woman, but it was more of an objective fact, but now with the knowledge that he was going to have sex with her, she was... breathtaking. He could see the way she was dressed, the way her clothes clung to her in just the right way. He felt his face grow hot and he averted his gaze, trying to focus on something else.

Caster couldn't help but giggle. She had wanted to see him look at her like this for so long, and now he finally was. It was addicting. She decided to give him a little taste of what was to come. She stood up from the chair, stretching her arms over her head, her breasts straining against the fabric of her dress. She watched his eyes widen as they took in the sight of her body. She took a deep breath, making sure to stick her chest out even further.

"What are you doing?" He croaked, his voice thick with lust.

"Just stretching. You know, before the big event." She winked at him, her smile turning mischievous. She turned around, and bent over to touch her toes, her dress riding up slightly and giving Shirou a tantalizing glimpse of her black stockings and the garters that held them up. He could even see a hint of white. Were those her panties? He swallowed nervously.

She turned back around and leaned over the table, her hands wound together behind her back, pulling in the opposite direction of Shirou, thrusting her chest out towards him. Shirou's eyes widened even further as he took in the sight. He felt his cock stir in his pants, growing hard from the sight of her full, round breasts pushed together by the stretch.

Caster took a step closer, her hips swaying gently with each step. She was playing a game of seduction, one that she knew she had already won this time. She could feel the heat radiating off of him, the desire that was building in his body. She took another step, and then another, until she was right in front of him. Without another word, she swung one leg over his lap and straddled him, her soft thighs pressing against his hips.

Shirou could feel the heat from her pussy, even through the layers of fabric separating them. He gasped as she leaned in, her breasts brushing against his chest, and captured his mouth in a deep, passionate kiss. Her lips were soft and warm, and she tasted faintly of mint, a pleasant surprise. They kissed for what felt like an eternity, their tongues dancing together, exploring every corner of each other's mouths. It was a kiss filled with longing and passion, one that neither of them had ever experienced before. "You taste so much better than it looked," Caster whispered against his lips before diving in again.

Their breaths mingled as they pulled away, both panting slightly. Caster's eyes searched his, looking for any sign of hesitation or doubt. All she saw was want, and it was all the confirmation she needed. She leaned in again, her mouth finding his in a gentle, sweet kiss that spoke volumes of her affection for him. Shirou's arms wrapped around her waist, pulling her closer, his hands slipping under her dress to feel the warmth of her skin. Caster moaned into the kiss, her body responding to his touch. She was lost in the moment, her mind racing with thoughts of what was to come.

Shirou's hands roamed up her back, feeling the smoothness of her skin, the curve of her spine. His thumbs brushed the undersides of her breasts, and she shivered with pleasure.

Her hips began to move in a slow, sensual grind against him, and he could feel her wetness through the fabric of her panties. His cock was now rock hard, pressing against her thigh, and she couldn't resist the urge to touch it. She reached down, her hand trembling slightly, and undid the button of his pants. She slid the zipper down with agonizing slowness, her eyes never leaving his.

Shirou's breath hitched as she reached into his boxers and wrapped her hand around his erection. It was hot and velvety, pulsing with life in her grasp. She gave it a gentle squeeze, feeling him grow even harder. Without another word, she slipped off his lap and onto her knees, the trail of her dress pooling around her.

Her eyes were wide with a mix of excitement and trepidation as she took in the full length of his cock. It was much larger than that man she had been forced to marry was. Even bigger than it had appeared in her memories. It was perfect, and it was finally hers.

Leaning forward, she took the tip of his cock into her mouth, swirling her tongue around the sensitive head. She could feel him twitch in response, and she took it as a sign of encouragement. She began to bob her head up and down, taking in more of his length with each stroke. Her inexperience showed in her tentative movements, but she was determined to make him feel good.

Shirou's hands found their way into her hair, guiding her rhythm. He was gentle at first, not wanting to overwhelm her, but as she grew more confident, he became more demanding. She took his cock deeper, her cheeks hollowing as she tried to swallow all of him. It was a tight fit, but she was determined.

Caster's eyes watered as she took him in all the way to the base, her throat working to accommodate his size. Shirou's grip on her hair tightened, his hips bucking slightly. He couldn't believe the sight before him: his beautiful, voluptuous Caster, on her knees, servicing him with all the love and devotion she could muster. It was more than he had ever dreamed of.

Her mouth was warm and wet, and she had a natural talent for it, even with her limited experience. She sucked and licked, her tongue dancing along the underside of his shaft, teasing the sensitive spot just below the head. Shirou groaned, his eyes rolling back in his head. This was heaven.

Caster could feel his cock swelling even more in her mouth, and she knew he was close. She increased her pace, eager to taste his release. It had been so long since she had felt this alive, this needed. The thought of finally being the one who got to bring him pleasure was intoxicating.

And then, just as she felt the first pulse of his climax, Rin's voice pierced the air. "What the hell are you two doing?!" she shouted, her cheeks flushed with surprise. Caster's eyes snapped open, and she jerked reflexively at the noise, slightly dislodging her mouth from his cock. His semen shot out, painting her in a warm, sticky arc. It splattered against her plump lips and began to trickle down her chin, tracing the adam's apple of her throat and spattering onto her breasts, which jiggled enticingly from the sudden movement.

Shirou's eyes went wide, his orgasm momentarily forgotten in the face of Rin's unexpected interruption. Caster looked up at him, her own eyes glazed over with passion, a smear of his cum glistening on her chin. She giggled softly, her cheeks stained pink with arousal. "It seems we've got an audience," she murmured, licking her lips clean of his seed.

Rin stared at them, her eyes darting between Shirou's still-twitching cock and Caster's flushed face. "You can't just start without me," she scolded, her irritation masking her arousal. "You have to orgasm at the same time! And I have to be here to initiate the transfer!"

Caster chuckled, standing up and straightening her dress. "Don't worry, little me," she said, her voice dripping with honey.

"Don't tell me not to worry!" Rin snapped, her eyes pulling her away from Shirou's cock to take in the sight of Caster. Caster had a smug smile on her face as she slowly licked the remnants of Shirou's cum from her fingers, savoring the sweet and salty taste. She enjoyed the jealousy and desire in Rin's gaze, knowing exactly how she felt through experience. Sorry, but he's mine this time.

"You just delayed the ritual! In case you forgot, you have to orgasm together! Now we have to wait for him to be able to go again!" Rin hissed, ripping her eyes from Caster to glare at the wall behind her.

"I told you not to worry," Caster repeated, a playful smile curving her lips as she snapped her fingers. In an instant, the sticky evidence of their passion disappeared, leaving her skin as flawless as before. She stood up with a grace that seemed to defy gravity, her movements a symphony of seduction as she began to disrobe. Shirou's eyes followed her every move, his arousal rekindling as if the brief interlude had never happened.

Turning to face Shirou, Caster bent over seductively, her firm, round ass pointing towards him as she slowly slid her panties down her thighs. The fabric whispered against her skin, revealing the glistening wetness that coated her folds. She stepped out of them and turned, giving him a full view of her bare breasts, the nipples hardened into tight peaks from their earlier play. She could not wait to finally feel him inside of her.

Without missing a beat, she straddled him once more, this time with nothing between them. He felt her heat and wetness as she lowered herself onto his cock, the sensation making him gasp. She moaned loudly as he filled her up, her walls stretching to accommodate his size. Her breasts bounced slightly as she settled into place, the sound echoing in the room.

Rin's cheeks burned with a mix of anger and desire as she watched her future self claim Shirou so brazenly. "You're such a slut," she murmured under her breath, but she couldn't deny the way her own body was responding to the sight. She knew she couldn't compete with Caster's confidence and experience, but she couldn't help feeling a twinge of envy as Caster began to ride him, her hips moving in a hypnotic rhythm that had Shirou's eyes locked on her bouncing breasts. Soon Caster would be gone and Rin would get to have him.

Shirou's teeth grazed Caster's nipples, eliciting gasps of pleasure from her lips. His hands gripped her hips, guiding her movements as he pushed himself deeper inside her. Caster threw her head back, her ponytails swinging wildly as she took him in, her moans filling the room. Rin's own arousal grew, a silent observer to the passionate scene unfolding before her. She clenched her fists, fighting the urge to reach down and give in to the need that was building inside her.

Caster leaned back, her breasts thrusting out as she worked herself on Shirou's cock. Her hands found his shoulders, her nails digging in slightly as she moved faster, her moans growing more urgent. "You're so...so...ahh!" she gasped as she reached her peak, her body convulsing around him. He felt so much better inside of her than it looked.

"You weren't supposed to climax yet," Rin grumbled as she squirmed at the debauchery taking place in front of her. "You have to orgasm TOGETHER!"

"Ahhh!" Caster moaned as she looked towards Rin without ceasing her bouncing. "Then... Then! My Sheeeeeeeero! Will just have to force me to cum againnnn!" She screeched.

Shirou's eyes flashed with a hint of amusement at Caster's challenge, and he tightened his grip on her hips, increasing the tempo of their coupling. His movements grew more intense, each thrust sending waves of pleasure through Caster's body. Her breasts jiggled wildly, the sight of which was almost too much for Rin to bear. Why was her chest even bigger than Sakura's? When would Rin get so large? Why hadn't she yet?

The room was filled with the sound of their passionate cries and the smack of skin on skin. Caster's breasts bobbed in rhythm with her movements, the sight of them driving Rin to the brink of madness. She clenched her fists and ground her teeth, trying to ignore how damp her panties had become.

Shirou's gaze remained locked on Caster's face as he felt her tighten around him. He knew she was close, her breaths coming in short, sharp gasps, her eyes squeezed shut in pleasure. He reached up and pinched her nipples, rolling them gently between his thumb and forefinger. Caster's eyes shot open, her pupils dilated with lust as she threw her head back and screamed out her climax.

"Shit, Shirou!" Rin exclaimed, her voice strained. She couldn't believe he had brought Caster to orgasm once more without reaching his own. Her hand hovered over her own crotch, but she forced it back to her side, knowing she couldn't indulge in such a way. "Why the fuck are you so easy, Caster?" She growled. "You have to orgasm together!"

Caster smirked up at Shirou, her eyes glazed over with pleasure. "What does that say about you, Rin?" She managed to get out between gasps for air. "You'll understand just how good this is one day, little me!"

Rin's cheeks burned with embarrassment, but she didn't have time to argue. Shirou had already pulled out and was standing before her, his cock still rock hard and gleaming with Caster's juices. "Take control," she demanded, her voice shaking. "She's clearly not capable of leading you to anything Shirou!" It was obviously just a demand for the sake of completing the ritual in a timely manner, not because that is what Rin would have wanted in Caster's place.

Shirou nodded, a smoldering look in his eyes. He didn't need to be told twice. He grabbed Caster's hips and pulled her to her feet, lifting her up with an ease that left Rin stunned. She wrapped her legs around his back, and he slammed her down onto his cock, driving it deep inside her again. She screamed out, her nails digging into his shoulders as he began to fuck her with a brutal rhythm that made the floorboards creak.

Rin's eyes were glued to the sight of Caster's large breasts bouncing in every direction with each thrust. It was a vision of carnality that was both disturbing and incredibly arousing. She couldn't help but let out a soft moan as she watched her future self's body respond so eagerly to Shirou's punishing pace. Caster's eyes rolled back in her head, her mouth open in a silent scream as she orgasmed for the third time in as many minutes.

Rin's own orgasm washed over her like a tidal wave, catching her completely off guard. She had been so focused on the scene before her that she hadn't even noticed her own building arousal. Her pussy clenched around nothing, desperately seeking the same release that Caster was getting. She bit her lip hard, trying to stifle her cries, but it was no use. Her body betrayed her, her legs shaking and her hips bucking as she came harder than she ever had before.

The room was filled with the sounds of their passion: Caster's moans, Rin's muffled scream, and the steady slap of Shirou's hips against Caster's ass. It was a symphony of desire that seemed to go on forever. And through it all, Rin couldn't help but feel a strange mix of anger and excitement. She was being outdone by herself, and there was nothing she could do about it. The man she loved, was fucking someone else, and she had no choice but to watch. And that someone else was herself.

Shirou's grip on Caster's hips tightened as he thrust into her deeper and harder. The veins on his arms bulged with the effort, his eyes never leaving hers. He knew that the transfer required them to climax together, and he was determined to make it happen. Caster's legs tightened around him, her heels digging into his back as she urged him on. She was close, so close, but she didn't want to come until he did.

Finally, it happened. The moment they had all been working towards. With a roar, Shirou spilled his seed inside Caster, and she clamped down on him, her pussy milking every last drop from him as she came again. The room was bathed in a soft light as the magic crest activated, the crest itself appearing on Shirou's bare shoulder and disappearing from Caster's. They both felt the intense burning in their right arms that should have made them cry out in pain. But instead they only felt the pleasure of each other.

Their breaths heavy, they stood there, his cock against her stomach, sweat glistening on their bodies. Rin looked down at them, a mix of awe and frustration on her face. "I-I'm sorry," she stuttered, "but we're not done yet. We need to transfer the rest of your circuits. Assuming you still want to. The crest could be enough..." The only person she was sorry for was herself.

Caster looked up at her, a smug smile on her face, "Don't worry Rin," she panted, "I'm more than happy to give Shirou whatever he needs."

Shirou, still trying to catch his breath, nodded in agreement. He would not deny that he didn't want to stop yet. Caster's body was a symphony of curves and sensuality that called to him, and her willingness to give up her magic circuits for him- to save people, was incredibly arousing. She had been a counter guardian, even if she wanted out now, she had given up her future for the chance to save people. He knew it was a cursed fate, but he still admired that Rin would choose this. Despite her claims, Rin was obviously a hero. He watched as she gracefully slid to the floor, her voluminous hair trailing behind her, and wrapped her ample breasts around his hardening member.

Her soft, warm flesh engulfed him as she began to rock back and forth, her eyes never leaving his. Each time the head of his cock emerged from her cleavage, she planted a gentle kiss on the tip, teasing him with her lips. The feeling was indescribable, a mix of pleasure and the promise of power that grew with each passing second. He moaned, his hands finding their way to her hips, guiding her movements.

The sight of Caster's tits bouncing with each stroke was too much for Rin to bear. She clenched her fists, her own desires rising despite her anger. She watched as her future self, the woman she would one day be, took such pleasure in serving Shirou. Her tits were even bigger than Sakura's somehow. It was infuriating and intoxicating all at once. But she knew they couldn't delay the ritual any longer. With a sigh of resentment, she stepped forward.

"Alright, you two, enough playing around," Rin snapped, trying to regain control of the situation, and to get this over with, so she could hide. "We need to do this properly."

Caster reluctantly pulled away, her breasts glistening with a mix of sweat and precum. She gave Shirou one last, lingering kiss before moving to her hands and knees, presenting her round, perfect ass to him. Shirou took his place behind her, his cock standing tall and ready. He reached down, grabbing her hips firmly, and pushed into her again. The room filled with the sound of their flesh meeting, Caster's moans growing louder with each thrust.

The sensation of being filled by Shirou was overwhelming for Caster. Each time he pushed into her, she felt herself stretching to accommodate his massive girth, her inner walls quivering with every deep thrust. She threw her head back, her ponytails bouncing with the rhythm of their union, her eyes rolling back in pleasure. She was no stranger to sex, but something about Shirou was different—his strength, his passion, his selflessness. It was intoxicating, and she couldn't get enough.

Shirou's grip on her hips tightened as he picked up the pace, his movements so rapid they became a blur. Her breasts swung wildly, slapping against her face and chest with each collision, leaving her breathless. She could feel the heat building within her, her pussy clenching around him, eager for release. And then it came, a powerful orgasm that made her scream out his name. But he didn't stop, didn't even pause. He just kept driving into her, pushing her closer and closer to the edge again.

Rin watched, her eyes wide with shock and arousal. She had never seen Caster like this—so wanton, so lost in pleasure. Her own hand slipped into her pants, her fingers finding her clit and moving in time with Shirou's thrusts. She bit her lip, trying to keep quiet, but it was useless. Her breaths grew shorter, her moans grew louder, until she couldn't hold back anymore. Her orgasm hit her like a wave, her body shaking as she came, her juices soaking her fingers. She felt guilty, watching her future self with such hunger, but she couldn't look away.

Caster's moans grew more desperate, her breaths coming in gasps as she felt another orgasm approaching. "Call me by my name!" Caster begged.

"Rin!" Shirou grunted, his eyes squeezed shut as he thrust into Caster, calling out her name as if he was lost in a sea of pleasure. Each time he said it, Rin felt a jolt of pain, a sting of rejection, but also an intense arousal that she couldn't ignore. Her hand moved faster, her own orgasm building like a crescendo. She could feel the wetness pooling in her panties, her body betraying her feelings.

Caster's eyes went wide with pleasure as she heard her real name on Shirou's lips. Her walls tightened around him, and she couldn't hold back anymore. "Yesss...Shirou...I'm coming!" she screamed, her orgasm ripping through her like lightning. This was so much better than watching him fuck this body was. He was so much better than she could have possibly imagined.

Shirou, lost in the moment, continued to pound into her, feeling her pussy spasming around his cock. "Rin...oh Rin..." he groaned, the name of both the girl he loved and his servant. Not the name of the girl who was his ally in this war, his classmate. The sound of his voice, calling out her name, sent a jolt of pure ecstasy through Caster, and she came again, harder than before. Shirou felt her walls tighten even more, her body shaking with the force of her orgasm. He couldn't hold back anymore. With a roar, he emptied himself inside her, filling her with his hot, thick cum.

Rin, watching the scene unfold before her, was barely cognizant of the world around her. Her eyes were glazed over with lust, her hand buried deep in her own pants as she felt the climax of her own orgasm crash over her. She could feel the magic in the air, the power of the transfer happening, and she knew she had to focus. With a deep breath, she forced herself to concentrate, to guide the transfer of energy carefully.

Caster screamed in pain, Shirou's cries joining her in a symphony, their pleasure forgotten as their bodies writhed in agony on top of each other, their legs intertwined. A magic crest was a layer on the surface, but this was different- a deeper process that ingrained itself in every fiber of their being- their very souls. Their bodies glowed as every single one of her circuits made their way into Shirou's body. Despite the howls of pain, Caster never stopped grinning ear to ear.

And then it was over. The pain subsided as quickly as it had came. "Hahahahahahahahahaha!" Caster cackled like a mad witch. The last time Shirou had heard her laugh like that- so untethered was when he had first summoned her. She had taken one look at him and burst out laughing like a madwoman. She had then dismissed her cloak- which Shirou hadn't seen sincev for some reason, and introduced herself as Caster. It was only later that Shirou would understand why seeing him was so funny- how could it not be to see a childhood friend as a heroic spirit?

Shirou's thoughts were cut off as Caster eagerly smashed her lips into his. Her tongue dove into his mouth, and traced the inside of his lips. Shirou was caught off guard, but eagerly returned the kiss. His penis began to harden again inside of Caster and she moaned into his mouth in response. His hips bucked reflexively.

"Stop!" Rin screamed. "The transplant is done! You don't have to do that again!"

For once Caster listened to Rin, breaking the kiss and looked at Shirou with the most brilliant smile, although she did not get off of Shirou's cock and rolled her hips. "It worked! I'm free!" She was silenced by Shirou eagerly claiming her lips to celebrate.

"You can't possibly know that!" Rin protested. "Not until your summoning ends!"

Caster pulled away from Shirou with exuberance. "I'm not a heroic spirit anymore, just an ordinary human! I get to stay here with Shirou forever!" Shirou bucked his hips as he leaned forward to kiss her softly.

Caster moaned in response as Shirou pulled away. "Caster, that's amazing!" He cheered. Forever.

Rin wanted to cry. What did she mean she was going to be here forever? She was supposed to be gone tomorrow! Shirou was supposed to love the Rin from his time, not the whore from the future! She was going to look like that too! She was the younger model! She wanted to be with him! She wanted to experience the feeling of his cock inside of her, his love. "You have no magical ability? Now what are we supposed to do about the grail? Your job was supposed to be destroying it while he dealt with Gilgamesh!"

Caster waved her hand dismissively as she bounced her hips against Shirou's. "Oh, that's easy. You'll do it."

"You were supposed to use your noble phantasm! I can't do that! And you don't have it anymore do you?"

Caster might have shrugged her shoulders, but it was hard to tell with the way that her entire body was shaking as she and Shirou began round three. "Oh that's eaaaaaasy! I'll j-just teach you to make a swooooord of your own!"

Rin stomped her foot in frustration. Had she just orgasmed again? Could those two stop fucking each other long enough for one conversation? It was rude! To not include her. "In case you had your brains fucked out of you, we have ONE night!"

Caster snorted. "So? I learned it in a few hours. I'm sure you'll be fine."

"Alright!" Rin cheered- she wasn't sure of how much of it was forced. "Get off of Shirou and let's go do this then!" A part of her was excited- creating the jeweled sword had been a goal of the Tohsaka family for years. And it gave her a chance to end this.

"When we're duuuuuhhhn!" Caster screamed as another orgasm wracked her body.

Rin stomped her foot again. "Fine!" She shrieked. "I don't have to watch this anymore!" But she wanted to. "I'll be in my room for when either of you remember THAT WE ACTUALLY ARE TRYING TO WIN A WAR!"


A/N:A lot of the early dialogue is ripped from UBW with minor changes.

If it wasn't obvious by the title and the fic itself, I opted to make this prompt subscribe to the bootstrap paradox. Or in layman's terms, the Rin that was summoned from the future is the exact same one as the Rin who is forced to watch as Shirou falls in love with her more experienced self. She spends a lifetime getting cucked, becomes a counter-guardians, and eventually gets her happy ending with Shirou! Worth it maybe?

Did this need to be a bootstrap situation? Probably not, but I love the concept of the bootstrap paradox and the requested more slutty Rin was hard to see as a realistic outcome- so it became her knowing what was coming in advance and basically just counting down the minutes until she got what she wanted for so long. And it was fun. I still don't know if it counts as NTR, but definitely has that vibe.

Given that Rin obviously killed Zouken shortly after she was summoned and it's a bootstrap situation, I definitely considered having Rin go ultimate cuck and become a CG to save her future self's kids. I ultimately left it very open ended on why she's here now and skipped an explicit how we got here epilogue scene of the Rin who got cucked's future because it didn't sound that enjoyable to write, and I wanted it more open ended.

Rin does actually say in the original VN that she would just transfer some of her circuits to Shirou but it would be a bad idea because she has to use them to transfer them and would be cutting off her supply midway through. It's because that option is unavailable that she gives up her crest. Given that having one crest is why Tokiomi gave up Sakura, and Rin moved them down to zero, I can not think of a bigger fuck Sakura if you choose this route moment. (You know, other than the fact that Sakura and Shinji canonically get married in the non-HF routes). Theoretically, with Caster there to facilitate the transfer, she wouldn't have to give up the crest. I strongly strongly strongly suspect that taking in a Heroic Spirit's magical circuits would be suicide- Archer's arm was literally the same person and caused Shirou to turn into a literal sword. But uh porn logic!

(On a side note, I want to write a smut piece at some point where UBW Shirou goes back in time to zero somehow (Probably the grail pulling him in or whatever), and still has Rin's crest. When he learns of Sakura's abuse, he goes to Tokiomi to try to get him to be decent and get his fucking daughter out of there, but he refuses because one crest or whatever. And Shirou mentions he has a second Tohsaka crest and offers it up if they keep Sakura. There's just one way to transfer it... and for the guy who gave up his daughter for a greater chance at the root, letting another man fuck his wife, Aoi is nothing. This is mostly because I hate Tokiomi and want to cuck him, but also the dynamic of Shirou pseudo-cucking Rin with her mother is hot. And obviously Shirou would end up replacing Tokiomi as Rin and Sakura's father in time because again fuck that guy.)

Obviously, fucking away your magic circuits is probably not a method to get free of Alaya, but I wanted a more happy ending and to add to current Rin's frustration. The thing that Caster mentioned talking to Archer about before he died was how to get himself out of the throne by doing the same. The next female Archer sees when he's summoned may or may not end up getting raped into taking all of his magic circuits. Oopsies, I'm sure the poor girl will be fine.

Rin's NP is the Jeweled Sword of Zelretch which is stolen valor. It's also a thing that is pretty distinctly at least somewhat hers. Shirou can only trace it onto her azoth dagger that's stored with all of her gems magical energy- not from nothing. He outright acknowledges that he cannot trace it, just imitate the shape onto the dagger he was holding. Rin can make it from scratch somehow at least in an impartial form when it is capable of utilizing the second magic in said limited form. And her failed version of a mystic code that uses the 2nd magic in a miniature jeweled sword can merge parallel worlds... And creates a singularity somehow? Her blatant use of the 2nd magic outside of what the sword is supposed to do is uh... impossible? Zelretch claims that he can't make Rin a magician because no seats are unoccupied but proceeds to compare her training to Aoko's training in magic... But like the Magics are all supposed to be unrelated and only able to do only their thing so how would he have made Rin a magician with an unoccupied seat? Then you also have Illya claiming that the Sword can only be used by Zelretch's lineage but Rin uses it and also creates her own later that she can also use so it's not just a Shirou's tracing thing...?

Anyways fun prompt, even if a bit shameless. Next prompt published will likely be another Rin face one this time with Ritsuka and very different vibes so look forward to it on Monday. (After this the next four won't have Rin). Leave Kudos, subscribe to me and this fic to not miss future prompt responses. Please comment more prompts even though I have over a hundred left to do, I like having options. Chronological order of submittance has very little bearing on what I do next, I choose what's vibing for me at the time.

Also if you want to give me prompts involving any of Alice Kuonji, Sion, Touko, Olga Marie, Kama, or Elizabeth Bathory; you have a good chance of getting it written sooner as I currently have zero prompts with less than five female characters for any of them and want to write them.