Muzan sneered, "You're right—a Lower Rank Demon is indeed not worth regretting. If he clashed with you and you killed him because of that, I wouldn't blame you.
But did you kill him because Pero clashed with you? No. It was because of Makomo, that human girl, wasn't it?
And tell me—when did you take a human as an apprentice? Why don't I remember? How many other things are you hiding from me?"
As the Demon King, Muzan could freely access the memories of his demons.
Until now, Ryuya's training with Kokushibo hadn't shown anything unusual, so Muzan had never bothered digging through his memories.
But this sudden incident—this unexpected variable—made him uneasy.
Anything he couldn't control… anything unknown… terrified him.
Muzan shoved his hand onto Ryuya's head and activated his Blood Demon Art, digging directly into his mind.
Ryuya grit his fangs. Under the overwhelming Bloodline suppression of the Demon King, he couldn't resist at all.
But then Muzan froze.
Why?
Why can't I access his memories? I can't find anything! Why?!
Muzan's eyes widened as he stared blankly at Ryuya, his mind racing.
Had his Blood Demon Art failed?
Or… was it because Ryuya was the offspring of Muzan and a human woman—different from demons created solely through Muzan's blood?
With his head bowed, at an angle Muzan couldn't see, Ryuya's lips curled into a cold sneer.
It was his Blood Demon Art shielding him from Muzan's reach.
Ryuya's Blood Demon Art sealed memories, prevented forced prying, and could create or delete memories at will.
It couldn't attack, but it was perfect for his situation.
And the existence of the System made up for its lack of offensive capability entirely.
"Ryuya, drink this!"
Enraged, Muzan sliced his finger, grabbed Ryuya's throat, and forcefully poured his demon blood down his son's throat.
Muzan's blood was something all demons craved—drinking it gave tremendous power, but also increased Muzan's control.
Ryuya swallowed it and felt a surge of strength, a refreshing force rushing through his entire body.
"Still useless? Why?!" Muzan, refusing to accept failure, force-fed him more and more blood.
Any ordinary demon watching this scene would be green with envy, mistaking this for a father's doting love toward his son.
After several attempts—each useless—Muzan finally stopped, panting with frustration.
He stared at Ryuya, his own son, with thinly veiled displeasure… and deep, gnawing unease.
Aside from being able to kill Ryuya whenever he wished, Muzan knew nothing about him.
Just like Tamayo.
Muzan hated this feeling. If looks could kill, Ryuya would be dead a hundred times.
"I'll let this matter slide—for now. If you dare to do this again, don't blame me for being impolite."
Although he couldn't pry into Ryuya's memories, Muzan pretended he had succeeded. Then he moved on.
"Turning a Hashira of the Demon Slayer Corps into a demon… who gave you permission? Explain yourself, Ryuya. And you'd better give me a satisfying answer."
On the surface, it wasn't a bad thing. When Doma first reported it, Muzan actually praised Ryuya's boldness.
If successful, Muzan could use this to locate the Demon Slayer Corps headquarters and wipe them out completely.
But when he tried sensing Flower Hashira Kanae Kocho's location… he realized something was very wrong.
He couldn't sense her existence at all.
Her memories, her presence—cut off entirely.
Just like Tamayo.
At first, he thought Kanae was simply killed. But after Ryuya's situation, Muzan reconsidered.
She was likely the third demon to escape his control.
Muzan's rage intensified, causing Ryuya to vomit blood nonstop as his skull felt like it would explode.
At that critical moment, Upper Rank One Kokushibo finally spoke.
"Lord Muzan. I instructed Ryuya to do it."
"What? Kokushibo, you allowed him to do this?"
Muzan's brows knitted, but his tone softened instantly, and the crushing pressure on Ryuya vanished.
To Muzan, all other demons were mere subordinates—but Kokushibo, Tsugikuni Yoriichi's twin brother, was a partner.
Someone Muzan respected deeply and trusted more than anyone.
"Yes," Kokushibo continued.
"But did this cause unnecessary trouble for you, Lord Muzan? I wanted Ryuya to hone his Moon Breathing Technique, so I had him turn the Hashira into a demon—seeking insight from other Breathing Techniques."
"So that's how it is."
Muzan nodded. So it wasn't Ryuya's idea?
But even so, Muzan would never tolerate another demon outside his control.
If he couldn't track Ryuya with Blood Demon Art, he'd surround him with demons to monitor him. Muzan refused to allow the unknown to exist.
"Lord Muzan, is there anything else? If not, I will take Ryuya with me," Kokushibo asked.
Muzan turned to Ryuya.
"Ryuya, I don't want to see this happen again. Without my permission, you cannot arbitrarily turn humans into demons.
"And since you killed Lower Rank Two, find a demon to replace that position. Quickly."
"Yes," Ryuya replied, glancing subtly at Kokushibo.
He knew perfectly well that his master had just saved his life. Without Kokushibo's intervention, Muzan might very well have killed him in anger.
And Muzan's life-or-death threat only strengthened Ryuya's determination to find Tamayo and break free from Muzan's control entirely.
"Ryuya, come with me. I have something to tell you," Kokushibo said.
Ryuya silently followed him, stepping out of Muzan's sight.
"Lord Kokushibo. Young Master Ryuya," Nakime greeted respectfully as they arrived.
"Please teleport us to my residence, Miss Nakime."
"Yes, Lord Kokushibo."
Nakime plucked her biwa string, activating her Blood Demon Art.
In an instant, the two were transported out of Infinity Castle.
Verdant bamboo swayed gently, bathed in pale moonlight that blanketed the quiet estate.
In the pavilion, a Go board rested under the moonlight, black and white stones scattered across it.
Kokushibo and Ryuya took their seats, beginning their master–apprentice conversation.
"Master—" Ryuya began, but Kokushibo raised a hand.
"No need to thank me. I'm not curious about your reasons, so there's no need to explain. Just… don't anger Lord Muzan again."
Muzan might forgive once or twice—never three or four times.
"Yes, Master. Did you bring me here to play Go?"
Kokushibo—once the eldest son of the Tsugikuni family—had always treasured both swordsmanship and Go.
Even after becoming a demon, his hobbies remained unchanged.
Before accepting Ryuya as a disciple, Kokushibo already played Go with him often.
After taking him in, he simply crushed him even harder.
"Play," Kokushibo said, pushing the bowl of white stones toward him.
Ryuya gave a helpless smile. He didn't even need to try—he knew he'd lose.
Playing against a centuries-old Go monster… if he won, he'd be the demon.
"Ryuya, your pace is still very slow."
After only a few moves, Kokushibo commented.
Ryuya needed time to deliberate before placing each stone.
Kokushibo, meanwhile, placed a black stone instantly the moment Ryuya placed a white one.
That was the difference.
"Master, I have to think. I can't just throw stones randomly and embarrass you, right?!"
"Thinking can be done in the instant before your hand moves. Just like when I taught you swordsmanship."
Kokushibo's words carried deep meaning. Ryuya paused, absorbing it.
Yes… every time Kokushibo played instantly, it wasn't that he wasn't thinking—he had already completed his thoughts before making the move.
Could it be—
"Transparent World," Kokushibo said quietly. "Have you heard of it?"