Yeah, so, I have never been able to find a Fate/Stay Night x Fate/Grand Order crossover that had Ritsuka travel to the world of Fate/Stay Night, despite Accel/Zero happening in F/GO itself. So, that lack of stories plus some inspiration from some Fate stories I recently read made me decide to do it myself. Hopefully, you'll enjoy!
It was cold.
With a groan, Fujimaru Ritsuka opened his eyes, his vision being filled with a cloudy night sky, a far cry from the ceiling of his room that should have been there. He sighed deeply at this revelation, only one phrase coming to his mind.
"It happened again," He muttered as his hands met his face, letting out a groan. What was it this time? A dream? A forced Rayshift perhaps? His hand rapidly went to his chest. He did feel fabric there, so he wasn't just sitting out somewhere in the middle of the night completely naked. Good. Glancing down, he saw the old reliable white fabric of his Chaldea Uniform. Not the worst Mystic Code to be transported in with.
Sitting straight up, he quickly took in his surroundings. He saw a baseball field and what looked like a modern school building in the distance. So he was in the modern day? If it wasn't for the blue lancer and red archer clashing in the middle of the baseball field, he'd swear it was his own school he had ended up at.
Wait, what?
Looking closer, Ritsuka realized that it was a fight he had seen many times through his few years at Chaldea, but it still was breathtaking to watch. The lancer, Cu Chulainn rushed ahead, jabbing at openings the archer Emiya left, who deflected each attack. They continued for a few seconds before the two stopped, talking a bit. Ritsuka sighed, he couldn't hear them from where he was. He tried his best to move a little closer, but froze in his tracks when he saw what Cu was doing.
The blue lancer slid into a stance, his red spear engulfed by a crimson red aura as he stared down the red archer. Oh, that was most definitely Gáe Bolg about to unleash its causality reversal effect. There was nothing Emiya had that could stop that version of Cu's Noble Phantasm. However, before Cu could unleash the attack, Cu suddenly snapped out of position, turning his head to the school and yelling,
"Who's there?!" The sounds of retreating footsteps filled the air, filling Ritsuka with dread. The way Cu reacted… well, there was someone here that wasn't supposed to be here. Now, Ritsuka wasn't a traditional mage, in fact, the last thing he wanted in life was to be a mage, but he knew most weren't good people. They'd likely kill this poor soul just for being in the wrong place in the wrong time.
Then again, maybe it was a spy, someone that needed to be gotten rid of. Still, he wasn't going to take the chance. He didn't have time to think about it anyways. Taking a deep breath, the boy yelled out at the top of his lungs,
"HEY! WHAT ARE YOU PEOPLE DOING AT THE SCHOOL THIS LATE?! I'M CALLING THE POLICE!" With that, both Lancer and Archer's head snapped in his direction, causing the Master to take a step back. He wasn't used to such deadly looks from those two directed at him. That was terrifying. Ritsuka quickly spun on his heel, rushing away from the school as he felt the gazes of the two Servants drilling straight into his back.
All he could do was run, despite the fact he knew it was foolish to believe for even a second that he could outrun either of his pursuers. Still, if he didn't make the effort, he wouldn't be able to save that poor soul that stumbled across the scene. For them, he'd keep running.
That wasn't the easiest thing to do, however, as the night made it hard to see through anything. All he could do is run down the quiet street, preparing for the worst while hoping for the best. It didn't take long before he could feel the hairs on his neck stand up, his instincts screaming at him that it was too late, this was as far as he could lead them. The Master of Chaldea stopped running, turning around and gritting his teeth, preparing to face down the two Servants, but when he turned, no one was there.
That couldn't be right. Surely, a person calling the police about this suspicious fight at a school should be higher priority than one person that happened to see them right?
"Hey there," Cu's voice called out from behind him. Ritsuka's eyes widened and he dove forward, tucking himself into a ball as he rolled away from the voice, quickly scrambling to his feet as he faced Cu, who stood there with his spear lazily held out in front of him, the red spear glistening under the sole street light that illuminated the road. If Ritsuka had turned around, his heart would have been stabbed by the cursed spear for certain. "Ooooh? It seems you have good instincts, kid," Cu complimented as he looked over the boy in front of him, a bored look on his face.
Ritsuka clenched his hands into fists as his mind rapidly began to process his situation and options while Cu was giving him some half hearted apology about killing the boy. If this was a Rayshift, then someone at the Wandering Sea would have tried to contact him by now. The fact they didn't meant something was really wrong, so it was unlikely he would be able to summon any Servants to his side.
However, he could feel his connections to all his Servants still, similar to how it was like in Shimosa, which would likely allow him to summon a Shadow version of them. The only issue was that a Shadow was many times weaker than a Servant. He could summon ten of them and Cu would destroy them like they were nothing. So his only option… was to try to confuse and scare the fearless man away.
There was only one person in Chaldea that could do this impossible task.
The back of Ritsuka's right hand began to glow as he stood tall and proud, ready to put on the theatrics. "Sorry, Cu Chulainn, but if you want to apologize, you shouldn't put some half assed effort into it. Same if you want to kill me, because I won't go down easy!" He raised his hand into the air. "Come forth, Guardian of the Dún Scáith!"
Cu's eyes immediately widened as he stumbled back, instinctively getting into a battle stance as he heard the title. A sort of mist came out from the back of Ritsuka's hand that slowly began to take shape, taking an almost perfect copy of the form of Scáthach, the only flaw being that the upper half of their face was engulfed in shadow.
"Impossible…," Cu muttered.
"Oh? What's impossible, Setenta?" Cu tensed up as he heard a voice he thought he would never hear again. "You have been summoned, correct? Of course I would come to see if your spear can reach me yet or not," The copy of Scáthach manifested two crimson spears, causing Cu to sweat.
"Dual wielding Gáe Bolg?" Cu muttered fearfully. "What the hell, Teache- wait a minute…," It was now Ritsuka's turn to sweat. Did Cu see through his trickery? But how? He knew he had perfected his voice impression of almost every Servant in Chaldea. Cu began to laugh, relaxing himself as he put a hand to his forehead. "Okay kid, I gotta admit, you had me going there for a second," He looked up at Ritsuka with a wild grin. "Wanna know what gave it away?"
Ritsuka thought to himself for a second before he realized it.
"Her bloodlust…," The Master muttered.
"Bingo. Anytime Teacher would want to fight me, she'd let out so much bloodlust all the ghosts in the Land of Shadows would scatter in an instant. Don't feel any from your little illusion there," Cu supplied. Ritsuka shook his head.
"Well, I'm not the only one who made a mistake here," Ritsuka fired back, causing Cu to blink. "Just because she's a fake doesn't mean she's an illusion," With that, the Shadow Servant dashed forward, thrusting its spear at the Child of Light, which Cu deflected with his spear, having to move back to avoid the thrust of the second spear.
"You're real interesting, kid," Cu said with a grin as he went to swing his spear at the copy of his teacher, which jumped straight up into the air, five Gáe Bolgs manifesting behind her and firing at Cu like arrows of death. "You know who I am," He continued as he dodged the spears. "And you know Teacher, not just what she looks like, but you're able to mimic her voice perfectly! That must mean… You've met her, yeah? But there's only one way anyone could meet her… you're a student of her's too, aren't you?" Ritsuka's blood turned cold when he heard that.
He had really messed up.
"... I've trained under her," He admitted, knowing that it was pointless to lie at this point.
"And now I'm really curious!" Cu said with a laugh as he dodged the Shadow Scátach's attacks, before landing a well placed spear strike into her chest, the shadow exploding into mist as the spear penetrated it. "Teacher's picky about her students, after all, and to take someone from the modern age, well… let's just say I wanna test you myself, as your senior,"
"Hey, hey now, come on," Ritsuka backed up slowly. "You're her greatest student, and I've been training under her for a little less than two years. I don't think it's fair to-,"
"Just consider this a pop quiz!" Cu called out as he jabbed Gáe Bolg at Ritsuka in a slow and obvious manner. Ritsuka had no issues dodging that strike, so slowly but surely Cu began to swing harder and faster, each blow getting closer and closer to hitting the Master of Chaldea. The boy could feel himself running out of breath as he was beginning to lose track of the spear's thrusts and instead started to rely on what he knew about Cu's habits to dodge the attacks.
"Oh yeah," Cu watched as he swung the spear, the strike deflected by a Shadow Servant appearing at the last second and vanishing just as quickly. "I never asked for your name,"
"Fuji... ," Ritsuka held his tongue as he ducked under a stab. "Fujimaru Ritsuka," He then felt the shaft of the spear strike him in the side of the head, causing the destroyer of Demon Gods and pruner of Lostbelts to tumble to the ground like a child slapped by an adult. The boy attempted to pick himself off the ground, only to have a spear pressed against his chest, the weapon not piercing his skin. He could see Cu smirking as he stood over him.
"Test over," Cu said playfully as Gáe Bolg dissipated into particles of mana. "Well, I hate to say it Ritsuka, but you are by far her worst student,"
"I coulda told ya that," Ritsuka muttered. "In fact, I was trying to,"
"Yeah, but this way was more fun, you know?" Cu chuckled as he observed the boy that was foolishly willing to lay down his life, yet acted with the confidence to survive despite that. "Still… I can see why she took you in," Cu smiled as he rolled his shoulder. "Well kid, it's been fun, but my Master's going to be expecting me to report back soon. Summon a Servant, yeah? We can have more fun when ya do,"
"Yeah, I might not be doing that if Archer kills me," Ritsuka muttered, looking around for any projectiles being launched. He knew that there was no way he'd dodge it in time, but maybe if Cu stuck around long enough...
Cu sighed and rubbed his neck.
"Sorry kid, but… he didn't come after you. He went after the one you were trying to protect," The Irish warrior apologized, sounding genuinely sorry this time. Ritsuka's eyes widened as he turned around, doubling back to the school. The Irish Hero sighed as he turned to leave, pausing as he looked over his shoulder, watching the boy disappearing in the night. With a final sigh, he turned into his spirit form, leaving the street empty.
The Master of Chaldea's lungs were burning when he made it back to the school, but he didn't let such an insignificant thing slow him down for now. He looked around, looking for some clue of where Emiya and his Master could have gone, when he heard some yelling from inside the school, along with a black and red glow coming from the second story window.
Ritsuka rushed inside, barreling through the school and rushing up the stairs. He had to get there, he wouldn't let someone die when he could do something about it! At the second floor, he ran around the corner, ready to tackle someone to the floor, but as he drifted around the corner, a flash of white caught his eye, causing the Master to twist his body in an attempt to move out of the way. However, trying to dodge at full speed caused Ritsuka to wipe out, the Savior of Humanity crashing into a wall gracelessly.
He let out a groan of pain as he tried to stand up, only to see the black blade of Kanshou pointed directly at him. The boy could only look up and see the cold, calculating eyes of Archer staring back at him, a sharp contrast to the warm and friendly eyes he was used to.
"You shouldn't have come back," Archer stated gravely as he raised Kanshou over his head. "Playing hero… will only get you killed," With that, the black blade swung down.