Basically where Kokushibo is in the place of Nezuko and Yoriichi is in the place of Tanjiro

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Michikatsu lie in the snow, breath becoming short, heartbeat becoming slow. 2/3 of his remaining family- that being he and his father, were attacked by something he couldn't even describe.

But it didn't matter- he was probably going to die. His father was already dead, succumb to the wounds. Michikatsu couldn't say that he's not glad... Though it seemed that if he's going down, Michikatsu would have to come with him. He hated his father, he deserved to die for putting him and his brother through all that abuse.

His brother. Yoriichi. He went into town today to deliver firewood. And he would come home to find his brother and father, bloodied and dead/dying... Michikatsu thought about how it could have been him here, dying, if he hadn't pushed the work he was assigned onto his kind brother instead.

➽──────────────❥

Yoriichi hummed as he pulled the empty cart up the hill, admiring the night sky. He was glad to do this work for him, he understood that he wasn't really in the best place at the moment... their father had been harder on him than ever, which made him uneasy to leave him alone with him. But, it was Michikatsu's request, so he left him be so he could keep reading.

As he came closer to the house, he scanned for his family members, but couldn't sense them... as he got even closer, he faintly smelled blood... as he arrived, he saw them.

Just by looking, he could tell his father was dead, but Michikatsu looked like he was still breathing slightly. He ran over to him, and checked his pulse. Brother is still alive! He quickly scooped him up, and started running back to the town. He hopped a doctor would be able to save him, even at this hour.

The cart was left far behind, which allowed them to take the quicker, but much more narrow path. Yoriichi felt his brother stir slightly, which he at first took as a good sign. But it quickly got more aggressive, knocking both of them off the small cliff.

They landed in the snow with a 'poof'- Yoriichi sat up as fast as he could, but for a few moments Michikatsu was nowhere to be found, at least not at first. He saw him, sitting like the samurai he wanted to be, much more imposing than usual.

"Michi, are you alright?" He started to run to him, extending a hand to try and help his brother up. That hand was met with a stone cold grip and piercing strength that he's never seen from Michikatsu before. He was thrown to the side as Michikatsu stood up, now getting a look at him. 6 red and yellow eyes adorned his face, and fangs were at the front of his snarl.

This wasn't his brother...!

➽──────────────❥ FOUR DAYS LATER

Yoriichi and Michikatsu had been brought to some little house by a person who claims to be a 'hashira'. Michikatsu had been knocked out for three days by this 'hashira', and because of the demonic transformation.

On the fourth day, Michikatsu woke up, and the twins had everything explained to them:

"What Michikatsu has become is called a demon.." The elder man said, lightly tapping his drum. This man's house was lined with books and drums, he was clearly a very creative person. While Michikatsu slept, the elder allowed Yoriichi to read a few of his own works. "And now that you two are here, you shall be involved with the Demon Slayer Corps."

"What's that?" Yoriichi asked.

"It's a small organization that devotes itself to slaying demons. Er- evil demons." He glanced at Michikatsu, who was slightly worried that he would be killed off. "Demons are creatures who feed off humans. Brutally murdering them to get their meals."

"I don't want.. to be a cannibal... ew." He interjected. Michikatsu was having a bit of trouble speaking since he became a demon.

"You're not going to. That's what makes thee demons evil- If you never eat humans, you'll stay good and avoid being executed. Anyways, I am now your trainer. My name is Kyogai. I already introduced myself to your brother, but you were asleep. I train people in the art of the sword-"

"Sword...?" Michikatsu immediately perked up.

"Yes, the Demon Slayers are swordspeople."

"So... this means father is gone... I get extra strength and power... PLUS I get to be a saumrai....!?!!!?!?!?!" He was bouncing up and down slightly, and smiling widely for the first time in a while. "I'm very... happy...!"

"..." Kyogai was silent for a few moments. "But... well, you're a demon. I'm sorry, but I can only teach your brother. You'd burn up in the sun if I tried to train you, plus I doubt that a demon would be allowed in the corps..."

"What..."

"I can see if-" Michikatsu ran to the small room that Kyogai gave him, slamming the door.

"Brother often isolates himself when he is upset." He said glumly. "...Is it true, that you wont teach my brother? He really wants to be a swordsman. In fact, he wants to be the best in all the land.. I don't wanna take his dream."

Michikatsu wasn't going to let that happen.

At first, he felt crushed. Did that teacher plan to keep him confined for the rest of who knows how long? He started to cry, his dream was floating above the palm of his hand, but was snatched away before he could even grab onto it.

'If only I didn't push my work off onto Yoriichi...'

And of course, the large mirror didn't help him feel better. The shock of seeing six eyes stare back at him, and the disgust that followed only made things worse.

But soon, he was able to put those feelings into achieving his dreams. That guy didn't want to teach him directly? So be it. He used his enhanced hearing to listen in on all of the lessons, taking detailed notes. In addition, he read all of Kyogai's writings on breathing styles and swordsmanship, even if they were stories.

Michikatsu had put together a comprehensive study guide on the basics of breathing styles, and the information he learned about Sound Breathing to create his own. First, he used his flesh to create his own sword, Kyokokukamusari. He fused the powers that the demonic transformation gave him with what he taught himself, creating Moon Breathing. ...He also learned to play the drums, which didn't really interest him. But it's another skill under his belt that Yoriichi didn't have, so he supposed it was fine.

He bid Yoriichi farewell as he was sent off to final selection. He could sense that finally, he was the stronger twin. Michikatsu ignored how that was likely do to the physical enhancements that being a demon brings, and mentally patted himself on the back for all his hard work. Over that week, Kyogai and him built a box in which he was supposed to be carried on his brother's back, should he return.

Michikatsu found this undignified and wildly demeaning, but at least they didn't force him to wear that mouth thing that the Hashira person wanted him to wear at first. He couldn't wait until they were fighting the real deal, until they were fighting evil so he could jump out of that box and impress his brother. Then, for his heroic deeds, he would be automatically accepted into the Corps and everyone would love him!

➽──────────────❥ A FEW MONTHS LATER

Except, that didn't happen.

He sure did impress Yoriichi, but that's where the truth of his imagined scenario ends. Once it had been discovered that he and Yoriichi were traveling together, they were nearly killed. Thankfully, they had just been dragged to the Corps HQ instead.

They were tried before the Hashira: some wanted them dead, some wanted them to live. An old man was crying in the corner, for some reason. One of them even stabbed Michikatsu, then forced him to prove that he wouldn't eat him.

"Why would I... wish to eat you...?" He asked. "Aside from the fact... that I don't eat humans... you're quite... ugly..." He stated.

"PERIODDDD!!! GET THAT WANNABE ARTIST'S ASS!" A voice shouted, seemingly from behind a bush. He was quickly muffled and pushed down, so Michikatsu couldn't tell for sure.

"Shut up! We're not supposed to be here, remember?" The one who pushed him down whispered.

"I need to show that fine babe my support! You'd do the same if it was your mid-looking red one."

"Firstly, I don't speak like that. Secondly, how can you call my man mid when they're literally twins!"

"The eyes make mine SO much more attractive. Plus it looks like he has bigger tits."

"Muzan! Stop staring at the tits of every man you look at!"

"You're one to talk, I know you're staring right at the red one's."

"I ammm!" She giggled.

"Erm actually Uta, you can't do that because blah blah blah yap yap yappatron." He imitated his friend.

"huh!" Uta gasped. "They just pushed my one to the ground!" She stared at Muzan for a few seconds. "You think he's into that?"

"How is your self-proclaimed twig body gonna push that guy onto the ground?"

"Girl I don't know, maybe people get a strength boost when they're horny or something."

"Maybe that's why I'm so strong!" Muzan rolled up his sleeve, flexing that arm.

"What are you horny for? The wall!?"

"NO, WHAT!? Even I'm not that weird. Anyways, speaking of walls, I just realized that if I want my man to be safe in my room, I'll need to get rid of the windows."

"How?"

"First, I'll make a new bedframe out of GALVANIZED SQUARE STEEL-"

"Why? In what way is that related..."

"So I can galvanize on his square 'till I steel, obviously!"

"WHAT??" She bursted out laughing, but still trying to keep the volume to a minimum. "Speaking of which, I should probably coat my bed in ECO FRIENDLY WOOD VENEERS, so he can eco friendly on my wood-"

"What wood, huh?"

"STOP REMINDING ME!"

"Shh," Muzan put a hand over her mouth. "They're issuing the sentence."

➽──────────────❥

Muzan and Uta had their sights set from the beginning. But they couldn't just go up to Michikatsu and Yoriichi respectively and tell them about the conversation they had behind the bush. They had to get to know them, and make them feel comfortable. 

Muzan, being Muzan, pulled some strings to get his way. The two had 'arranged' (waited at the front door of the Spider Mansion and talked official to make it seemed arranged) for their shared rooms. Muzan did in fact remove the windows to his room, but didn't reconstruct the bed. No renovations were made to Uta's bed, either.

"So, this will be your room now!" Muzan shut the door then set down the box, making sure no sunlight would get into the room before opening it up.

"Thank you... I greatly appreciate.. you not forcing me.. to stay in that disgusting... little prison..." Michikatsu returned to normal height, stretching after hours of being carried in that thing.

"No trouble!"

"Uh..." Michikatsu said after a few seconds of looking around the room, Michikatsu's eyes feel on one corner. "There is... only one bed...."

"Oh!" Muzan forgot that most people don't like sharing a bed with a near-stranger. "We uh... didn't have the funds!"

"I see... sorry if... my comment was offensive...."

"No, It's fine! It's fine, please, make yourself at home! I'm going to get something quickly, I'll be right back.

Michikatsu looked around Muzan's room. It was medium sized, with one bed, of course. In the opposite corner of the bed, there was a tall desk with a tall spiny chair. On the desk were a bunch of tubes, beakers, and liquids. Michikatsu didn't know what they were for, nor could he figure out what the big thing in the center of Muzan's desk was. It was kind of tube-y, and it had small little balls next to it and inside of it. He wondered what it did...

➽──────────────❥

Meanwhile, with Muzan and Uta:

"So, how did not buying another bed go for you?" Uta asked as Muzan sat down across from her.

"Fine, I guess? I lied and said that we were too poor to buy another. I don't think he's all that into it."

"HA L!"

"Excuse me?"

"I called mine cute, and he smiled a little. I think he blushed too! After that, he didn't even mention the bed." 

"I bet you're gonna find him sleeping on the floor."

"No, I'm not!" (She did.) "Anyways, just praise your buddy six eyes, I don't think either of them got enough as children..."

"That's FOUL. But thanks for the tip, I suppose."

"My tip is reserved for my future husband."

"And that's where I end this conversation, as I will not feed into your delusions of you having a dick." He laughed, walking back to his, and now Michikatsu's, room. 

As he walked through the halls, he took note of the setting sun. Perhaps he and Michiakatsu could go outside. It could be a date! He really needed to get to know him, anyways. Deep down, he knew they couldn't have a relationship just because he thinks he's hot.

Muzan carefully opened and closed the door to their room, making sure to not get any of the remaining sun rays on Michikatsu. He walked over to the bed, and stared down at him. Michikatsu was lying on his back, hair down, hands folded on his upper stomach. His expression was monotone.

"Hey cutie, what're you doing?" He asked.

"My name is... Michikatsu... not... 'cutie'..."

"I know that, but it's a nickname! I think you're cute, so I called you an affectionate nickname to show that."

"Oh. Sorry... anyways, I'm just... thinking... about death, mostly..."

"Wow..! That's chill, that's chill..." Muzan was concerned, but tried to be sportive and brush past it. He didn't know what type of death he was thinking about, so he decided to let it be for now. "Anyway, the sun is setting. When it's fully dark, do you wanna go outside and hang out? Get to know each other?"

"That sounds........ fun..." He said, smiling ever so slightly. He didn't know why, but Muzan had been so incredibly kind to him. And not in an annoying way like Yoriichi was, either. Nobody besides Yoriichi has ever wanted to spend time with him, and nobody's ever given him an affectionate nickname.

"Is it a date, then~?" He winked.

"Today is-"

"No, no!" He chuckled. "I meant as in going on a date with someone to see if you wanna be in a relationship with them!"

"Oh.. I see..." Michikatsu looked away in embarrassment at his mistake.

"It's okay, your little misunderstanding was sweet!" Muzan sat down on the bed.

"Thank you..." He sat up, his and Muzan's legs now almost touching. "It shall be a date..." Michikatsu said, smiling at Muzan's proposal.

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2550 words

I've had this au idea for a while, but I scrapped the original fic a few months back. I'm much happier with this version! Maybe I'll do a part two, where Muzan and Uta actually develop a relationship and romantic feelings instead of staring at mantits. (They're on their way, but I didn't really explore it much in this chapter.)