What happens when you summon Emiya Shirou in Chaldea

Shirou vs Muramasa

Shirou let out a small sigh. "Sorry about this, Muramasa."

Muramasa let out a groan. "It's fine...but seriously, how do you keep ending up in shenanigans like this?! And just for the record, I'm a blacksmith, not a fighter!"

"Blame both Da Vinci and curiosity from...well, a lot of servants."

"Figures..." Muramasa grumbled. "Who was the one that started the entire thing?"

"I have no clue."

Currently, both Shirou and Muramasa were in Shirou's reality marble, and they were not alone. In the distance were Ritsuka, Mashu, Gudako, Fou(who had no interest in the fight), Arturia and her counterparts, Illya and her counterparts, Miyu, Irisviel, Jaguar-Man, Kiritsugu, EMIYA and his counterpart, Okita and her counterpart, Musashi, Merlin, and surprisingly, Ciel and Da Vinci.

Musashi looked at the two excitedly. "Finally! The biggest question that has been on my mind will be solved today! Who will win? Gramps or Shirou-kun? This will be exciting!"

As for why Shirou and Sengo Muramasa are having a sparring match...let time rewind for a bit.

(A few days ago...)

"Hmmm..." Musashi hummed as she was meditating in her room, thinking about something. After a few seconds, she gave up and laid back on the ground. "Argh! It's useless! I can't get the question out of my mind!" she yelled as she looked at the ceiling in front of her while flailing her arms and legs. Eventually, she stopped moving her arms and legs and stood up. "Maybe I should take my mind off the question by doing something else instead...that should work at least, right?" she thought to herself.

Turns out, what was on her mind earlier still persists, no matter what she does to forget it.

"Jeez! No matter what I do, it is still there! Really...why am I being bothered by the question of who is stronger between gramps and Shirou-kun?! It should be obvious that Shirou-kun could win! But...gramps has that punch of his and he pulverized a person by cutting him in half! Hmm....gah! I don't know who is going to win! This is so frustrating!"

"Um...is there something wrong?"

Musashi nearly jumped before turning around to see Okita Alter, who was munching on a stick of oden. "Oh, it's you, Okita Alter. Don't scare me like that."

"Sorry. By the way, it seems like something is on your mind. Is there something wrong?"

Musashi let out a sigh. "Well...let me ask you a question first. Who do you think can win in a fight? Gramps? Or Shirou-kun?"

"Gramps? Who is that?"

"Muramasa."

"Ah. Now I get it," Okita Alter said as she finished her oden. "Well...about that question...I have no idea either. I mean, I have never seen him fight before. You have before, haven't you Musashi?"

"Yes, but not that much though...argh! It's so frustrating! I want to know the answer right now!" Musashi said.

"Why not ask Muramasa then?"

"That's the problem! He's stuck inside of Shirou-kun's reality marble! And the only way to access it is through Shirou-kun himself!"

"Then why not ask Shirou?"

"He's busy at the moment! And I don't want to disturb him!"

"Hm...that is a good point there..."

Both Musashi and Okita Alter were stumped at the conundrum as they tried to figure out a way to solve it. Luckily, an unexpected person came at the right time.