A month prior to the first meeting with Michikatsu, Muzan met with two other demons in a private study within the Dimensional Infinity Fortress, both of whom were among his informal group of elite and powerful demons. The first was a young man around Muzan's physical age with dark red hair, and the second was an older man who appeared to be in his fifties with black hair. Both looked eager to talk with Muzan, and as he sat down to talk with them, he asked, "So, I heard you two have made progress on your mission. What kind of progress are we talking about?"

"A week ago," the younger-looking demon explained. "I received a tipoff from a demon whom I had taken under my wing as per your instructions. She had apparently ran into a woman known as Tokito Tenko and her young twin sons. She looks fairly normal, so she was able to fool this woman into thinking she was human."

"Tokito Tenko, eh?"

"She had just lost her home recently, so this demon let her stay within her own home. She was, of course, about to eat them before she revealed who she used to be married to."

Muzan was quite interested in this turn of events. "Go on."

"She was once the wife of Tsugikuni Michikatsu. Her sons were fathered by him. As such, she let her go since she could gather information by talking to her. She later left after a week of staying there, so I assume she left today."

Muzan gave a faint smile. "Excellent. Tell me, what was she able to gather?"

"She hasn't seen the older Tsugikuni twin in four years," he informed him. "Not since he abandoned her and her children to become a demon slayer. She also reported that based on what she heard from her, the older Tsugikuni twin has somewhat of an inferiority complex towards his younger twin brother, Tsugikuni Yoriichi."

This information piqued Muzan's interwar most of all. "An inferiority complex… Now that sounds interesting and easy to exploit."

"I completely agree," this younger demon replied.

"Allow me to interject," the older demon then added. "I'm currently building a group of humans of my own, and it just so happens we're based not too far from where we believe the elder Tsugikuni is living."

"A group of humans? How curious…"

"I guess you could call it a cult," this older demon explained. "Humans are so, so easy to exploit. If they're desperate enough, they'll believe anything you tell them. I'm surprised more demons don't try this."

"It isn't a bad idea," Muzan admitted. "Of course, it carries risk, but if you can get humans to bow before you, you can get a good insight into society even with the fact we can't go out at night. I want you to use your cult to spy on the Tsugikuni brothers as much as possible. They won't suspect humans one bit."

The younger demon asked Muzan, "I must ask, master. What do you intend to do with them?"

"If I can get one of them to join us," Muzan explained. "We'll be able to win. Think about it. One of the most powerful demon slayers joining us would give us a massive advantage. Of course, this is merely the first step of the plan."

"And the next one?"

"Spread demons beyond Japan. In one years' time, I will begin my journey to China via the foreigner ships coming to Japan. From there, I will return here to prepare for my next set of trips. I want to spread Japanese demons as far as I can. Now, Junpei, Katsumoto, I need you two to focus all of your intelligence gathering on those two brothers."

Junpei, the younger one, replied with a nod, "Yes, master."

Katsumoto, the older one, replied, "As you wish, master."

"Now, go do it. I have some private business to attend to with Tamayo." Once both of the demons he had summoned disappeared with a snap of his fingers, Muzan smiled to himself. "This will be my greatest triumph."

Present

Two days before Michikatsu was to meet with Muzan, he had decided to come clean to Yoriichi about the meeting. The two discussed their options at Michikatsu's home, with Yoriichi eager at using the meeting as a way to confront Muzan once and for all. "I want you to meet with him first," Yoriichi planned out with him. "Pretend to be on his side, you know? It's clear that he thinks he can give some level of trust in you."

"I'm not sure why," Michikatsu remarked.

"Maybe he thinks you'd be easy to kill if you betray him."

Michikatsu sighed. "Maybe… Look, Yoriichi, this whole thing's been really getting to me."

"I know," Yoriichi replied as he put a hand on his shoulder. "As my older brother, I have a duty to be by your side. We'll get through this together, nii-san."

"Thank you," Michikatsu replied with a nod and a faint smile.

"Now," Yoriichi then said as he got up. "I have business to attend to. Stay safe and keep me informed, nii-san." As he walked out the door of his house, he paused and told him, "I know we're both adults now, but… I love you, nii-san. I don't want to see anything bad happen to you."

Michikatsu replied solemnly, "I love you, too, Yoriichi." Once he was gone, Michikatsu battled the temptation of agreeing to work with Muzan to keep his wife and children safe. "This whole thing's messed me up," he thought to himself. "Do I protect the corps and my brother, or do I protect Tenko and my children? Can I do both? Is that even possible? I'm still having these nightmares of Tenko dying horribly, and they're getting worse day after day after fucking day." He would battle with these conflicting thoughts for the next two days, only making a decision on the day of the meeting.

On the day of the meeting, both Yoriichi and Michikatsu set out to complete their task. They had told nobody else about what had transpired due to the fear of the latter being accused of treason by not informing the corps as soon as it happened or by not fighting Muzan hard enough. Yoriichi set out to take a long way around to find the area Muzan said he would be at so Michikatsu could get to him first. However, his plan to only be a few minutes behind Michikatsu would be derailed once he found himself in a forest clearing.

As he walked around the clearing, he could hear footsteps that did not exactly match up with his own. He turned around to find two men standing behind him. "Hello there… Tell me, who are you two?"

"We're demons," Junpei explained to Yoriichi as he displayed a naginata to him. "And I can also use a blade. It may not be a katana, but I can still take you down."

"We know that your brother has set out to meet our master," Katsumoto informed Yoriichi as he seeped a visible red cloud of gas from his mouth. "And we also brought company." He then turned behind him and shouted, "Come out! I command you as your god!"

On cue, about ten humans emerged from the forest, encircling Yoriichi. All of them carried various blunt weapons with them. Some looked confident in their mission, while others looked reluctant to carry it out. Yoriichi, shocked at what was before him, asked Katsumoto, "So you brainwashed these people?!"

"These people worship me," Katsumoto explained. "I am their god! Now, my beloved followers, attack him!"

All ten humans jumped onto Yoriichi at once. He did not want to kill humans, but realized immediately that he had no choice in the matter. He used his sword to impale one of the humans, the blade going all the way through his stomach, before pulling it out and kicking a second human down and then stabbing him in the chest. One of the cultists shouted in fear, "What the fuck?!"

Another one shouted, "We need to kill him for our god! We can't give up now!"

Yoriichi then decapitated two humans at once before he was hit in the back with a large stick, knocking him several feet before he jumped up into the air and decapitated a third human. One of the remaining humans shouted, "This guy's fucking crazy!"

Yoriichi shouted back as he slashed him across the chest before slicing his neck open, "You're the crazy ones!"

A surviving follower shouted to the others, "Get him! Don't just fucking stand there! Kill him!" Yoriichi replied by decapitating him, causing the last three cultists to drop their weapons and run away in fear.

Yoriichi shouted to them, "Run! Run away and don't look back! You're free now!"

"So," Katsumoto said. "It's just us now?"

"You bastards… Making me fight other humans like that was sickening! I'll dispatch you two faster than you can feel it!"

Junpei then charged at Yoriichi with his naginata. Yoriichi replied by fending him off with his katana, the two holding firm as they traded blows several times. Junpei narrowly missed Yoriichi's neck as the latter jumped up and slashed his right shoulder, causing him to drop his naginata briefly before he picked it up. "Fuck, that hurt!"

"Breath of the Sun, Second Form: Clear Blue Sky!" Yoriichi delivered a circular horizontal slash, managing to cut Junpei in half but miss his neck, causing him to mutter, "Shit, I missed!" Just then, Katsumoto jumped onto his shoulders, pushing him to the ground before he exhaled a gas into his face, which caused his eyes to water and burn in pain. "Agh, shit! My eyes! My eyes!" Yoriichi pushed Katsumoto off of him and slashed his arm off.

"Damn it, my arm!" Katsumoto yelled at Yoriichi, "Just give up already!"

"Not a chance!" Yoriichi then charged at Katsumoto before jumping into the air, declaring, "Fifth Form: Setting Sun Transformation!" While upside down, he delivered a killing blow to Katsumoto, cutting his head completely off and ending his life in front of a shocked Junpei. As he landed on his feet, he told Junpei, "Wanna keep fighting?"

"Ah… Of… Of course!" Junpei was able to snap out of it and charge at Yoriichi. "Take this!"

"Third Form: Raging Sun!" Yoriichi then deflected Junpei's naginata attack with two powerful horizontal slashes and blowing him back a full five meters. "Now for the final attack! Ninth Form!" As he charged at Junpei, his slashes became even more powerful, and Junpei could feel wind from the sword's maneuvers through the air. "Dragon Sun Halo Head Dance!"

"Oh no! No-"

Yoriichi then decapitated Junpei after slashing his body five times before then, effectively cutting it into many pieces. Now without a head, he could not grow back from his individual body parts even if he wanted to. Yoriichi looked at the death he had caused all around him and shook his head, telling himself, "This didn't need to happen today… That one demon with the cult was truly monstrous…" With Junpei and Katsumoto out of the way, Yoriichi would begin his trek to meet Muzan once more, though his delay would prove to be deadly.

Meanwhile, as Yoriichi battled Junpei and Katsumoto, Michikatsu waited for Muzan in the exact same spot he had seen him several days before. However, he did not intend to go through with his brother's plan. Instead, he had a plan of his own.

As Muzan once more materialized in front of him, this time accompanied by a young woman, he greeted Michikatsu with a friendly tone. "I'm glad to see you've returned, Tsugikuni."

Michikatsu, curious as to who the girl was, asked, "Who's she?"

"This is my associate and partner Tamayo," Muzan replied. He then turned to her and shot her a glare, commanding her, "Say hi to our guest, Tamayo."

"He- Hello…" Tamayo seemed timid and almost reluctant to say anything. "Ni- Nice to meet you, Kokushibo-sama…"

"Kokushibo-sama?"

"I figured that if you're going to join me," Muzan explained to Michikatsu. "I might as well give you a name to use. After all, if you become a demon, you won't be using your human name anymore, at least not officially. You can still use it with your wife and kids. I don't care about that."

Michikatsu took a deep breath as he prepared himself mentally to talk to Muzan. "Okay, so… Kibutsuji-sama, do you promise to help me protect Tenko and my children?"

"I promise," Muzan assured him with a seemingly warm smile. "I'll put them on a no-kill list. In fact, if they ever decide to become demons, I will gladly help them. I won't force it on them, though. That's something for you and your family to decide. By the way, I'm glad you're already referring to me as your master."

"Thank you, Kibutsuji-sama," Michikatsu replied, now fully willing to join Muzan and become a demon since his question had been answered and he thought his family would be safe. "I will gladly become a demon and join you."

"I'll tell you what," Muzan then told him. "So normally, I don't let demons say my name for security reasons, but… Since you're my most valuable asset, I can cut you a deal."

"A deal?"

"Before I make you a demon," Muzan explained. "I want you to kill the head of the Demon Slayer Corps, any of your brother's students, and as many other slayers in your way as possible. If you can do that, I'll let you call me by my name once I demonize you, as long as you acknowledge me as your master when you do so."

Michikatsu knew that he had to do it. He knew that to fully gain Muzan's trust, he would have to fight his own colleagues and kill them, maybe even his own brother. "I see… I accept those demands, Kibutsuji-sama."

Muzan then placed a hand on Michikatsu's head as he bent down on his knees. "I hereby declare you to be Kokushibo. From now on, you will serve me and demonkind. I expect great things from you, Kokushibo. Do not disappoint me."

"I will not disappoint you," Michikatsu assured him. "I will kill whoever you wish for me to kill."

"Now," Muzan then commanded him. "Find your former leader's house and kill him first."