Disclaimer: Naruto (High school version) - Fate/Stay - I own neither
I have been hinting at the crossover on my discord for awhile now.
Story Start!

The Holy Grail.

The granter of wishes, and goal for mages everywhere. It was the pinnacle of magecraft, and wars had been waged for the right to have a wish granted by such an object.

7 Masters.

If there were not 7 Masters, then the Holy Grail itself would pick suitable master candidates for the ritual. Masters who would be guided towards their wish or their ultimate end, a challenge for many, but the reward was enticing to say the least.

"... and done."

Uzumaki Naruto, graduated high school student and current collge student, 18 years old and 180 cm tall, a fine height for a Japanese man, and the body of an athlete. He wore a loose white shirt and blue jeans, and he was holding a silver dagger in his hand. Below him was a large magical circle, glowing underneath him.

He had been chosen.

The proof of him being chosen was on his right hand, a light symbol of a red spiral with a stroke of red simples above it and below it. He had been chosen by the Holy Grail to play the role of Master, and the entire thing had been explained to him over the phone by a priest. He was to take part in a Holy Grail war if he so decided to.

He decided to.

"Archer, Saber, Assassin, and Caster are all that remain... Saber sounds awesome." Naruto looked at his item that he would use in his ritual. A simple way to connect him to a hero who he would summon. Still, he wanted to summon something strong enough to be useful to him, and he had no relic to use.

So, he did some reading.

He would summon using his own cursed blood as the item, as he was related to the legendary Tamamo no Mae herself through his blood. His blood would be the key that allowed him to summon a powerful servant, and the best way to use his blood would be to fill a sake cup with it. He didn't imagine she would be a Saber, she seemed like she would be a Caster or maybe an Assassin since she worked covertly when she caused nations to war against themselves.

He slashed his forearm.

His blood dripped from his arm and filled the sake cup.

There was a chant he was suppose to say.

"YO MOTHERFUCKER! Wake the hell up, I'm your Master, and you're my Servant, so answer my summons and come forth! Come be my partner, and take part in a hell of a ride!"

Not the chant.

Fuck the chant, he said the REAL chant in his mind, while he shouted at the summoning circle that glowed brightly. He didn't like that formal kind of magecraft, since it was just so weak compared to the old days when his great ancestor was alive. He would say what he wanted to say, and let his servant know what was up before she was even summoned.

There was a glow as his servant's body materialized before him.

She was a female though.

She was not a kyuubi or a fox at all, and that was clear. If anything, she looked like the famous voice actress Aoi Yuki... but with horns, and more colorful. She looked more like a skanky Aoi Yuki, legendary sensual voice actress. She had most of her body on display, though she was flat chested for the most part. She had some sexual legs though, and she shamelessly had her kimono open to show the lightest amount of armor over her crotch and breasts.

She picked up the blood filled sake cup and started to drink it.

Fangs.

"Assassin. Shuten-Douji." She laughed into the sake cup as she drained the blood out of it with a sensual moan, and she had a voice that was smooth like butter, and as intoxicating as wine. "Please accept my gratitude for summoning me. I will do as I please but... you don't mind, do you?" Shuten Douji herself extended her foot and touched him.

He took a deep breath.

"Only if I can cut your head off if you go too far."

"... I like you." Shuten stated as she looked at her Master, who was giving her an amused look, and showed that he knew of her legend, and how she had died. She touched her neck gently, and she leaned her head forward in a tilt. "... and what does my Master... Master... how about Husband... or maybe Lord... I could call you Sweetie too."

Every word she spoke was dripping with sexual energy.

A lesser man would pop a boner at the sound of her voice, and them jizz in their pants at the fact that she was talking to them. Instead of doing either of those though, Naruto sat down on the hotel floor and just looked at her.

"Call me what you want... but tell me what you want from the Holy Grail... or whatever you want to tell me."

"Ufufufu... I have no specific wish for the grail... but I will enjoy watching you. Ah... yes... good looking... I do enjoy good looking guys... especially those with blond hair, and blue eyes." Shuten stated as she looked up her well built master. A grown man who towered over her in height, even when both of them were sitting down. The blood of a blond haired, blue eyed man, in a red sake cup had been the perfect catalyst for summoning her into the Holy Grail war.

How could she resist such a tempting summoning?

She couldn't.

"What do you like... I like good food and good fights."

"I enjoy good food, good drink, good sex, and good fights." Shuten agreed with what Naruto was saying, and she added on her own spin. Her words were all spoken slowly as she looked him up and down even more, and her grin turned from malicious to gleeful. She had a sexual malice to her smile, but now her smile seemed far more gleeful. "... and my Sweetie... what is your desire for the Holy Grail?"

Naruto grinned.

Shuten smiled.

"I'll use it to enslave you... can't have you bringing yourself back to life, and killing people can I?" Naruto told her what he was going to use the Holy Grail for outright. He had no wish for himself that he would use the grail to grant, but he could make sure that if he won, that Shuten wouldn't be able to take advantage of the grail either. "... if you're going to do whatever you want... then I'm going to do whatever I want. In the end... we're a Servant and Master."

"Should I get a choker then?" Shuten teased.

Naruto smiled at her.

"... We're going to get along just fine, and when you betray me-"

"-which I might do." Shuten agreed that she would possibly betray him in the future. "I might eat you... or eat you depending on how much I like you by the end of this... I do like handsome men with fight in them." Shuten tilted her head even further to the right, and curled her long legs up to her torso and licked her lips. "... and will you use your Command Spells on me... to make me do what you want?" Shuten asked as she looked at the command spells on his hand.

She could attack him and cut his arm off.

Naruto laughed at her.

"Fine." Naruto slowly started to stand up, and he extended his arm towards her. He gave her a look and dared her to cut his arm off, because he could see that look on her face that showed she had thought about it. He was giving her the chance to attack him and end their relationship as a Master and Servant.

Shuten gained a glint to her eye and stood up, looking up towards him with a dangerous smile, showing that she was daring him to use a command spell on her.

"Ufufufu... my my... you are just fun, aren't you. I do... I truly do like you Sweetie." Shuten could not help herself but wait for the order. He was calling her bluff by making his command so slow that she had ample time to cut his arm off and kill him. He was bluffing her by telling her that he would waste a command spell on her.

"Shuten Douji, by my command spell..." Naruto started off slowly, and he stared her in the eyes and dared her to do something to him. Her smile widened and she showed off every tooth in her mouth with it, and she didn't do anything as she watched with baited breath. She wanted to see how far he would go.

Masters only had 3 command spells to use.

Only 3.

"Make it count... or I might be able to ignore it... I might get angry." Shuten stated as she leaned forward and gave his arm a little bite, putting her teeth on his forearm and allowing them to sink in. He just watched her and called her bluff, not backing down from her teeth even as she drew blood from his arm. "... shall our Master and Servant relationship end right now... with neither of us winning... you dying... and me vanishing?" Shuten dared him.

He didn't flinch.

He stared right into her eyes, and the command spells on his hand started to glow even brighter as he charged an amazingly high amount of mana into them. He did it SO SLOW that she could have easily bit his arm off at any given moment.

He called her bluff again.

"Bite my arm off... I dare you... come on... I'm about to command you to do something... you can bite my arm off right now and stop me if you want." Naruto stared into her soul with his words being spoken. Shuten let go of his arm, and she sat down on the floor with a her smile becoming more soothed.

The subtle malice was gone.

Naruto lowered his arm, and Shuten watched as the bite mark she left behind healed at a shocking speed. A regular human would not be able to heal at that speed, and she tasted his blood on her lips.

"... I do like you..."

"Good... and since I summoned you... lets win together, and we can decide if we're allies or enemies when all is said and done." Naruto extended a hand towards her. She looked at his hand, and she poured herself a cup of sake, before she extended the cup towards him instead of her hand.

"A drink."

The second she said that, she pulled the cup away from him and started to drink it. She moved her body across the floor towards him, and he leaned back when she crawled into his lap and grabbed his shoulders. She grabbed onto his shoulders tightly and got up onto her knees so that her head was above his own.

She opened her mouth and allowed sake to roll off of her tongue.

"A drink... lets have fun... win or lose... kill or be killed... lets enjoy ourselves... lets kill the enemies... or be killed by our enemies." Shuten watched as the sake dripped over his face, and he looked up towards her. "... I am an evil oni... come Sweetie... let us enjoy each other's company... like demons... as is our blood." Shuten called him out on being a descendant of Tamamo no Mae.

Naruto sighed and ran a hand through his hair.

He sure got lucky with his servant.

Since they were in Japan, he wanted to summon Tamamo no Mae since he was her descendant and her kyuubi blood flowed through his veins. What was more was the fact that in Japan, the fame she had would make her an even stronger seravnt, and him being her descendant would make her much more likely to be loyal to him without the need for using any command spells. It was too bad that the method he used for summoning her, seemed to have also been capable of being a catalyst for Shuten Douji as well.

It just so happened that he was her type of man as well.

Naruto pushed Shuten off of him.

"Fuck it, we're going to a bar... I've got a fake ID we can use. Come on Shuten." Naruto grabbed his college blazer and tossed it over her body, so that she could put it over her kimono and be decent enough to be seen in public. Shuten wrapped it around herself casually, and grinned at how he was going to buy her modern day booze.

"I do like you Sweetie... you are just my type."

"Come on, lets get you drunk off your ass, and explore the city... might as well take a tour, since once every servant gets summoned, we won't get the chance to look around." Naruto stated as he grabbed her by the horn and moved her head from side to side a few times. "... and who knows, we might meet another Master tonight... take out the competition early."

Sadly, that didn't happen.

They ended up spending the entire night at the bar.

Purely by coincidence, a man who attempted to rape a woman in an alley nearby the bar was found dead the next morning completely drained of blood... and bones. Completely by coincidence, and not at all because a blond haired Master walked out of the bar and saw what was happening, and just so happened to let a hungry Shuten punish the man in a way that was fitting of what he had tried to do.

Pure coincidence.

Chapter End!
Please Leave Me Lots of Nice Long Reviews, But No Flames Please!

Shuten is short and petite, but she is NOT a "loli" in any way, shape, or form.
The joke is that she has the same body type as her voice Actress Aoi Yuki, who her design was based off of. She has the body of a petite ADULT woman, and you insult Shuten by calling her a loli, then you are body shaming her voice actress for having that body type.