Yoriichi arrived just over half an hour after Michikatsu had left. Indeed, waiting for him was Muzan and Tamayo, but no Michikatsu. As he approached Muzan, the two locked eyes, and Yoriichi could feel something within him telling him that his whole life had led up to this very moment. "All my life," he thought to himself as he drew his sword. "I've been searching for the reason I was born. Well, I think I've found it."

Muzan thought to himself, "So this is the legendary Tsugikuni Yoriichi… He certainly looks just like his brother." He turned to Tamayo and told her, "Stand back and let me do the work."

"A-… Are you su-?"

He then yelled at her as he also glared at her, "That's an order! Stay the fuck back!"

Tamayo, frightened by his demeanor, stepped back and replied, "Yes, master."

Yoriichi then told Muzan, "Who's that girl you're with?"

"She's my partner for the time being," Muzan explained. "She won't be relevant to us."

"Where's my brother?"

"He's already gone," Muzan smiled. "It's just the two of us. If you want to find out what he did…" He then calmly rolled up his sleeves to bare his arms to his opponent. "You'll have to fight me first."

Yoriichi replied, "You better tell me where he is, Kibutsuji Muzan. You know for a fact that with breathing styles, you're more vulnerable than ever before."

Muzan laughed in response, amused by Yoriichi's boast. "That's great… You think I still care about those stupid little breathing techniques or styles or whatever the fuck you call them?! Look… I'm no longer interested in swordsmen who use breathing styles."

"You will be soon."

Muzan replied to this last quote by charging at Yoriichi at an inhumanly fast speed, faster than any other demon he had ever fought as Muzan also let out a loud and deafening roar. However, Yoriichi was able to dodge this attack, and as he regained his footing, he could hear the trees behind him snap and fall. "Get back here!"

"Holy shit," Yoriichi thought to himself. "Muzan… Muzan's more powerful than I thought! Even one scratch could kill me!"

"You won't be so lucky next time!" Muzan then charged at him again, but Yoriichi was able to deflect him with his sword, shocking Muzan. "Wait, what?!"

As Yoriichi backed off, he could sense that Muzan was no ordinary demon at all, and had a body structure meant to make him almost impervious to being killed. "I can sense he has multiple hearts and brains scattered through his body," he thought to himself. "I still don't know how many, though!" Muzan let out another loud roar as he charged at Yoriichi again, who called out, "Twelfth Form: Flame Dance!" He dealt a vertical and then horizontal slash to Muzan, and as he did, he could now get a clearer picture on how many hearts and brains he had. "Okay," he thought to himself. "He has seven hearts and five brains. Mere decapitation will not kill him."

To Muzan's horror, his wounds healed much slower than usual after Yoriichi's counterattack. "What the fuck?! Why?! How?!" He clutched at his chest wounds as they slowly healed for a few seconds before shouting, "How did you do that?!"

"Maybe you just aren't that powerful."

Muzan, enraged by Yoriichi's insulting comment, charged at him again. "You'll pay for those words!"

Yoriichi saw the opportunity and seized upon it. In an instant, he decided that now was the time to premiere a new attack of his that he felt would be enough to deal the critical blow he needed to finish Muzan once and for all. "I must strike now," he thought to himself as he waved his sword in the air. "This is my one chance!" He then yelled, "Thirteenth Form!"

"Thirtee-?!"

Muzan was then cut off as Yoriichi began to continuously attack him and cut away at his body, with any of his counterattacks being quickly intercepted and stopped. "All twelve forms are in use," Yoriichi thought to himself as he continued to strike at Muzan. "I can see the damage I'm doing!" Indeed, with every slash came more and more blood that fell onto the ground and coated the bamboo trees around them in a dark shade of red as Yoriichi continued to hack away at Muzan, who was horrified and unable to resist in any meaningful capacity.

Tamayo, who was watching on the sidelines, expressed in shock to herself, "He… He might actually kill him!"

Muzan screamed as the continuous attack went on, "No! This can't be it! No!"

Finally, after completing the continuous attack, Yoriichi backed away from a bloodied and badly wounded Muzan, who clutched at his neck and shook in fear. "Before I kill you," Yoriichi then demanded of him. "I want to know two things… First, where the Hell is my brother, and second… What is the value of a life to you?"

Muzan refused to give an answer to either question as he continued to bleed from his wounds. Instead, he thought to himself, "The sheer audacity of this man to ask me questions after doing this to me… This fucking bastard… This… Fuck him! He won't be so high and mighty when he finds out about his brother joining me!" His face grew red with anger as Tamayo looked on with hope in her eyes. "Not even Tamayo is helping me… That bitch!"

Tamayo thought to herself, "Could this really be it?! Could I finally be free?!"

Yoriichi told Muzan as he stepped forward, "Answer me."

Muzan instead bared his teeth at him in unbridled rage and screamed, "NEVER!" He then proceeded to explode in a massive flesh-filled explosion, causing hundreds of small pieces of his body to fly out, most of which Yoriichi cut away and dodged. The explosion also kicked up dust and a lot of smoke, and when it cleared, Muzan was gone, but clearly not dead due to the fact that some pieces had managed to escape.

Yoriichi sighed and said to himself, "For fuck's sake…" However, he then hears crying and whimpering behind him, and turned to find Tamayo on her knees. "Huh?"

"You almost killed him…" Tamayo was almost ripping her own hair out as tears poured out of her eyes. "I was almost free of him… But… That motherfucker evaded decapitation! Not even beheading can fucking kill him! God damn it, I wanted that piece of shit dead! I wish Kibutsuji Muzan had died by your hands!" She then gasped as she stopped crying, realizing she had uttered his name out loud. "Wait…"

"Did you just say Muzan's name out loud and not die?"

"I… I did…" Tamayo was in a state of shock. "Why did I not die?"

Yoriichi decided to spare Tamayo, seeing that she hated him just as much as the Demon Slayer Corps hated him. "Don't be afraid of me. I'll let you go. Just… Tell me what you know."

Tamayo gave a faint smile to him and told him, "Thank you… That man has taken everything from me. I was sick, and he pretended to be a doctor. He offered to cure me of my illness so I could be with my husband and children longer, but instead, he turned me into a demon and I didn't even fucking know it." Her smile went away as she continued her story, her eyes filled with mourning and despair. "As with any demonization, I was mindless and not in control when I was turned into a demon, so I…"

"You what?"

Tamayo began to cry again as she covered her face with her hands. "I ate my husband and my children! When I realized what I was doing, it was too late!"

Yoriichi felt terrible for considering to kill her earlier, despite knowing he was supposed to. "I'm so sorry."

"That bastard then made me his slave," Tamayo continued. "I was made to serve him however he wanted, and I couldn't say no to him, lest I be killed."

Yoriichi then sighed and decided to make her an offer. "Since you're free of Muzan's control apparently, how about we do this: I'll let you live with me, and you'll help me kill Muzan."

"Live with you?"

"I won't force you into anything you don't want," Yoriichi assured her, knowing that that was a concern for her due to her past with Muzan. "Just don't kill people while living at my house unless it's in self-defense or the defense of others."

Tamayo wiped away her tears and thought to herself for a moment before nodding. "I can accept that. Just to warn you, though."

"What is it?"

"Muzan will probably never show his face to you again," Tamayo told him. "At least, not willingly. I could sense genuine fear in him, which I've never felt before. He's scared shitless of you."

Tamayo then got up, being helped up by Yoriichi. "Thank you. Now… There's something you need to know about your bro-"

They both then heard footsteps nearby. Yoriichi turned to her and told her, "Hide! Quickly!"

"Got it," Tamayo replied before quickly running off and finding a hiding spot in the trees.

"Yoriichi! Yoriichi!"

Yoriichi could see that Sora was running towards him with another slayer in tow. "Sora?! Sora!"

"Yoriichi, hurry up and come here!"

When the two met in the middle on the path, Yoriichi asked him, "What is it?! What's going on?!"

Sora, clearly shaken up and in distress but not physically hurt, told him, "There's been an attack!"

"An attack?!"

"It's your brother," Sora told him as he caught his breath. "He attacked your students!"

…

Earlier

Having run a full 15 minutes after meeting Muzan to get to Yoriichi's house, Michikatsu approached the front door and was immediately met by his crow, who flew down from a window above. "Michikatsu! Michikatsu! Welcome ba-"

Michikatsu slashed the crow in half immediately, killing him instantly. As he saw Yoriichi's crow fly off into the night sky, he muttered to himself, "Shit, I won't be able to stop him. Oh well." He thought to himself, "I know I have to kill my former leader, but I have to make a stop here first."

"Hey," then said a slayer as the door opened, revealing a man bearing a sword on his hip. "What's going on out he-?"

Michikatsu immediately stabbed him in the chest, his sword going completely through him. "Nothing personal. This is just business." He then calmly stepped over his body after pulling his sword out of him and walked into the house. "Kibutsuji-same will surely like this…"

From another room, two younger students of Yoriichi, both aged fourteen, ran out of the sleeping quarters for his students. One of the two, a girl with white hair, asked Michikatsu out of concern, "Michikatsu-sama, what were those noises outside? Is there an attack? What are we going to do?"

Michikatsu silently looked at the two of them and simply drew his sword, startling both of them as they then saw the body lying on the ground behind him. The second of the duo, a boy, shouted, "Run! Run! He kill-"

Michikatsu decapitated him and the girl at the same time in one swing, their heads falling to the floor as their bodies went limp. He then ran into the sleeping quarters where Yoriichi's other seven students were still either asleep or just waking up due to the noise. One of them tried to reach for his sword, but Michikatsu sliced his abdomen open while he was still on his bed before pulling a knife out from his belt and stabbing another girl who tried to run past him, running his knife through her three times before she fell. Another student yelled, "We have to go!"

Michikatsu replied by grabbing him by the shoulder, pulling him in, and then slicing his neck open before using his sword one-handed to decapitate a student who again tried to run past him. The remaining students screamed in fear, and one tried to climb out of a window, only for her to realize she was stuck due to it being too small. "Come on! No! I can't be stuck! I ca-"

Michikatsu ended her life by ramming the sword completely through her. The four-foot long sword ended up coming out through her mouth partially before he pulled it out, allowing some of her insides to spill out onto the ground before the last two kids ran out of the room successfully. "Oh no you don't," Michikatsu yelled as he chased them out of the house and onto the road out front.

"Get away! Please, stop!"

"Never," Michikatsu yelled as he finally caught up to both of them, ramming through one with his sword but failing to stop the second. As he ripped his sword out of the young boy, who was no more than twelve, he looked on at the one that got away and sighed. "Oh well… It's fine. I've done my damage anyway." He then calmly walked back to the house and once inside, he found a lit candle that had managed to remain undisturbed throughout the entire slaughter. "Time to burn this place to the ground." He then tipped the candle over, allowing it to catch the floor on fire before he walked out. "Now to the Oyakata-sama's house." As he did, he dropped off a letter he had written earlier in the day in front of the house, which would soon after be found by Sora.

He had killed eight of Yoriichi's nine students, who ranged in age from just ten years old to nineteen years old, all of whom aimed to be demon slayers and learn the Breath of the Sun before setting alight what was perhaps one of the most important buildings in the corps aside from the Ubuyashiki Estate. While this alone would have sent shockwaves throughout the entire corps and severely hurt it due to the loss in potential manpower and a prime training site, what he was about to do next would shake it to its core for generations afterwards.

…

While Sora raced to the Ubuyashiki Estate to hopefully warn them in time, Yoriichi raced to Michikatsu's house to try to find Tenko and their twin sons. As he ran through the forest to reach his house, all he could think about was the fact that his own brother had betrayed the corps. "This can't be real," he thought to himself. "How… Nii-san, how could you betray us?! How could you betray me?! This has to be some kind of mistake, some kind of deception by Muzan!"

About twenty minutes after being told of Michikatsu's betrayal, he arrived at his house and slammed on the door several times. Tenko, who was asleep, immediately got up and walked over to open the door, wiping her eyes and yawning as she did. "Ugh… Hang on!"

When she opened the door, Yoriichi immediately asked her, "Has my brother been back here at all since he left here earlier in the day?!"

"Wait, what's going on?"

"He…" Yoriichi knew that telling her would break her heart, but also knew she had to know the truth. "Your husband betrayed the corps and killed my students. Rengoku Sora found this letter written by him."

Tenko could not believe what he had just said, and quickly snatched the letter from his hands to read it. "Let me see that!"

Everyone,

I am no longer Tsugikuni Michikatsu.

I am Kokushibo, a servant and soon to be demon of Kibutsuji Muzan.

I did not make this decision lightly, and all I ask is that no harm come to my wife or my children, regardless of my actions.

Thank you.

Tenko exclaimed in shock, "What?! He what?! Where is he now?!"

"I don't know for sure," Yoriichi replied. "But I have a feeling he's heading for our leader's house."