What happens when you summon Emiya Shirou in Chaldea

What is the Tsumugari Muramasa

"Hmmmmmmm..." Okita Alter said as she was laying down on the couch lazily while looking at the ceiling above her.

"What is it Alter me?" Okita asked with a sigh.

"Well...I've been thinking about something."

"What is it?"

Okita Alter let out a small sigh before answering her non-altered counterpart's question. "I wonder what happens when Shirou uses Tsumugari Muramasa."

Okita was confused with her altered counterpart's question. "What do you mean by that?"

"Think about it. Shirou is a saber-class servant, which means he uses a sword as his noble phantasm, well one of his noble phantasms, correct?"

"Well yeah, but what does that have to do with your question?"

"Well, it got me thinking, what does Shirou do when he uses Tsumugari Muramasa? Every of the saber-class servants that I've seen would have their noble phantasms either be having a giant beam from the sword or a sequence of sword strikes. However, I don't know what kind of attack Shirou's Tsumugari Muramasa is. Even during that time where all of us witnessed Tsumugari Muramasa in action, we weren't able to see it because of the pillars of flames."

Okita let out a sigh. "Really? That's the thing that was on your mind?" she asked.

"Yeah. Doesn't it bother you too?" Okita Alter asked.

Okita scoffed. "No, it doesn't," she replied. "Although she does have a point. Even after Shirou-san's demonstration of Tsumugari Muramasa, I still don't know what kind of attack it was. All that we know of is that it's an anti-reality noble phantasm that can purge resentment, cut through bonds, fate, causality, and destiny, and slice through the idea of karma itself. Not to mention the fact that it can destroy a reality marble itself. Just what is it?"

As both Okita and Okita Alter were thinking about it, it was then that a servant came into the room they were in.

"Hey there Okita! Okita Alter!" Musashi exclaimed happily.

"Hm? Oh, hey there Musashi-san!" Okita said.

"Hey there Musashi..." Okita Alter said.

"So, what were you two talking about?" Musashi asked as she sat down on a nearby chair.

"We were talking about what the Tsumugari Muramasa is," Okita Alter replied.

"Eh?! One of Shirou-kun's noble phantasms?!" Musashi asked excitedly

"Yes."

"Okay. Why were you talking about it?"

"Because I really want to know what kind of attack it unleashes. Musashi, your noble phantasm unleashes a beam right?"

"Yeah, and your noble phantasm is a flurry of slashes with a beam from your sword in the end, right?" Musashi asked.

"Yes. The same goes for my counterpart as well, minus the beam part. Anyways, this is where the problem begins. I don't know Shirou-san does with Tsumugari Muramasa. I mean, what happens when he swings his sword? Does a beam come out? Or is it something else that I have never heard of? Doesn't bother you Musashi?" Okita Alter said.

Musashi then gave Okita Alter's question and long thought and said, "Well, now that you mentioned it, you have a point. Back in Shimousa, I don't know what kind of attack Tsumugari Muramasa was when gramps used it. To be fair, I didn't see it back then. And when Shirou-kun used Tsumugari Muramasa, we weren't able to see what it was because of the blinding light from the clash between him and the arrogant so-called King of Heroes."