Muzan lay sprawled across the top of one of the many rooms within the expansive Infinity Castle. Countless chambers stretched out in every direction, creating an eerie and boundless maze. Standing before Muzan was Kokushibo, his presence reminiscent of the way Yoriichi had confronted him in the past.
Overwhelmed with dread, Muzan's tears flowed freely as he faced the realization that his own Upper Moons would be the agents of his demise. Never in his wildest imaginings had he considered that he, the immortal demon, could be brought to such a fate.
Along with the tears in his eyes, he says, "I never thought that I'd be killed like this, this is even worse than hell. Have I lived so long just to die like this?"
Helpless and resigned, Muzan found himself unable to do anything but watch as his own fate unfolded before his eyes.
Seeing Muzan crying, Kokushibo says–
"Muzan-Sama, I won't kill you. Even if I lost my humanity, I still won't betray you. You gave me a new life otherwise I would die. If you want to kill and absorb me then do it. I have no purpose to live anymore," says Kokushibo in a calm voice.
Akaza grabs Douma's back head and bows down to Muzan.
"If you want to absorb someone then it should be this b*tch," says Akaza to Muzan.
"No, I am not gonna absorb anyone. There is another way to live again, but it will take decades. When the time comes, I'll absorb one of you. But simply absorbing y'all grants me nothing but some extra time to suffer," says Muzan in a severe tone.
"Woah, would that be me?" says Douma in a sarcastic way.
"Yes so? Do you have any objections? Or are you trying to tell me what to do?" says Muzan in anger.
"Oh my, that's so harsh," says Douma again in a sarcastic way.
Muzan lays out his plan to them. He intends to transfer his powers to the Upper Moons, giving more than half of his power to Kokushibo. Akaza will receive more power from Muzan than Douma, making Akaza stronger than Douma. As a result, Kokushibo becomes even more powerful than the demon king himself. However, Muzan will be absent for decades.
Muzan then detonates his body cells, essentially committing suicide. Despite this, he is not truly dead. Although he no longer has a physical form, he continues to exist within the cells of his Upper Moons and can communicate with them at any time. He plans to return soon.
With Muzan gone, Kokushibo ascends to become the new demon king, taking the place of the previous demon king, Muzan Kibutsuji.
After that day, Kokushibo's sole purpose was to train himself and fulfill Muzan's wish by making him immortal. However, he eventually realized he could never surpass his younger brother, Yoriichi.
About 2 months later, Kokushibo arrived at Oyakata-Sama's previous residence at night, finding it empty.
Suddenly, a glowing figure emerged from the darkness and approached Kokushibo.
"Is that… a ghost?" Muzan asked Kokushibo from his cell, where he resided with the remaining Upper Moons.
As Kokushibo tried to identify the figure, the white glowing figure appeared in front of him. His eyes widened in disbelief. It couldn't be but it was. Yoriichi was standing before him with that smile and Hanafuda earrings again.
Both Muzan and Kokushibo were shocked.
After staring at each other's eyes for a while, Yoriichi says, "Brother, sorry for being late. I will come after 50 years to save you, and eradicate Kibutsuji Muzan."
After saying that, he vanishes.
Muzan is extremely terrified, while Kokushibo is surprised yet excited at the same time.
"50 years later? Well, I'll be waiting for you," says Kokushibo.
"He doesn't have to be as powerful as he was in his previous birth, he must be weaker in his next incarnation. I'll surpass you this time for sure, Yoriichi," says Kokushibo to himself.
"There will be a day, we'll meet again."
Kokushibo never felt so excited before as he is right now. With the power of the demon king, he will reach his full potential soon.