Five days earlier, just after the fight was over...
Akaza, Upper Moon Three, found himself summoned inside the Infinity Castle, Muzan's residence.
(If we've been summoned here... This means an Upper Moon has been killed by a demon slayer.)
The demon threw his body down, and he landed shortly after in another part of the Infinity Castle. Suddenly, a vase appeared out of nowhere next to Akaza. Strangely, that vase spoke.
"Well, well, if it isn't you, Akaza-dono!"
From the vase, the demon inside of it appeared -- Gyokko, Upper Moon Five.
"So lovely to see you in such good health! What has it been, ninety years?" said Gyokko. "The thought that you might've perished made my heart sing!"
Then, another demon spoke up. "In the time we've been apart, you even forgot how to count, Gyokko. It's been one hundred and thirteen years."
It was Hantengu, Upper Moon Four.
Suddenly, from behind, Akaza felt someone placing a hand on his shoulder.
"Hey, Akaza-dono, how's it going?" It was Douma, Upper Moon Two.
"Get off me, filthy asshole," Akaza snorted, punching Douma right in the face, causing him to lose his jaw. "You know I don't like you."
However, Douma's jaw quickly regenerated.
"Always so violent, Akaza-dono... But that was some punch..." he muttered. "I can tell you're a bit stronger than before. Aren't you, Akaza-dono?"
Then, Nakime -- the one who summoned the Upper Moons, also called the Biwa Woman -- spoke up.
"I summoned Upper Moon One earlier than you all. He's listening even now."
Actually, in a not-too-distant room of the Infinity Castle, there was Kokushibo, Upper Moon One, and the strongest demon after Muzan himself. "Lord Muzan has arrived."
Suddenly, Muzan appeared.
"Gyutaro is no longer with us. This means the Upper Moons are now incomplete," said the Demon King.
"Is that true, my lord?" asked Douma. "Then I apologize, since I'm the one who bought Gyutaro in. How can I atone for this embarrassment?"
"No need to apologize. I knew Gyutaro would be defeated," said Muzan. "Daki proved to be his weakness. And do y'all want to know who killed them both?"
"Who killed Gyutaro and Daki, my Lord?" asked Douma.
"It was... Goku Tsugikuni," grunted Muzan as if saying that name caused him a knot in the tongue. "The son of the man who almost killed me five-hundred years ago..."
"Goku Tsugikuni? Isn't he the one who fought Akaza-dono four months ago?" asked Douma.
"Yes, it's him," said Akaza. "He managed to keep up with me -- even if I was chilling that day. It was expectable two weaker demons wouldn't kill him."
"Oh, I'm very curious to face this Goku one day, then!" exclaimed Douma.
"Don't be a fool, I'll be the one to kill Tsugikuni!" exclaimed Akaza.
"Shut up, all of you!" yelled Muzan. Everyone froze.
"I'm so pissed off... I'm so fucking pissed off right now..." muttered Muzan, shattering his bottles with potions with just his mind.
"Please, my Lord, forgive us!" prayed Hantengu.
"You and Gyokko did nothing useful in the last few years," said Muzan. "I'm starting to wonder why you even exist..."
"Please, Muzan-dono! We'll be the ones to kill Tsugikuni, I promise!" said Gyokko.
"Mpfh... I'm not counting on you," Muzan grunted. "But if you want to try, I won't replace you if you die..."
"I want to come too! I want to see this Goku!" exclaimed Douma.
"Did Muzan-dono give you any orders?" said Akaza, slicing Douma's head in half. "Get lost."
But then, suddenly, Akaza felt a presence that scared him. Kokushibo had arrived.
"Akaza..." he whispered, as Akaza's blood froze. "Tsugikuni is my nephew. I will be the one to kill him."
"Oh, you're right, Kokushibo-dono!" said Douma, regenerating his head. "Goku is your nephew, after all..."
"Tsugikuni is not the only one who needs to be killed," said Muzan. "You have to think about Kamado Tanjiro as well."
The Demon King pointed at Gyokko and Hantengu. "Gyokko, Hantengu, I send you. Kill both Tsugikuni and Kamado. I won't tolerate failure."
"As you wish, Muzan-dono..." muttered Gyokko.
"You're all dismissed," said Muzan. "One day, Goku Tsugikuni, I'll have you in the palm of my hand... And when that day comes, I'm going to crush you."
A little flashback from several centuries ago...
On the edge of a house, a man was sitting. That man was Yoriichi. Behind him, Yoriichi heard a man's voice.
"I brought you some tea," he said. That man looked a lot like Tanjiro.
"Thanks for the tea, Sumiyoshi," said Yoriichi. Yoriichi was holding a baby in his arms -- but that baby wasn't Goku. At the time, he had already given him to Tamayo.
"Listen, Yoriichi..." said Sumiyoshi. "How are you supposed to pass your Sun Breathing if you have no heirs?"
"I do have a heir, Sumiyoshi," said Yoriichi. "You know that. Somewhere, in this world, my son is probably sleeping in my friend's arms... I'm sure raising a child has to be one of the best sensations ever..."
"Oh, yeah, I always forget," said Sumiyoshi. "But what if your son doesn't manage to grow up or become a demon slayer, as you planned?"
"I'll pass my Sun Breathing to you, Sumiyoshi," said Sumiyoshi. "Even if I have 100% trust in my son's future. If there's one thing I regret in my life, it's not being able to raise and educate my child as I would like to... But for his sake, that's the better way..."
"I understand you, Yoriichi," said Sumiyoshi. "Your son will not fail, he'll become as strong as you, in a pair of centuries, when your friend will leave him live his life!"
"No," said Yoriichi, resolved. "He will become stronger than me. I'm sure of it, I just know he will."
"I'm sure of it too, Yoriichi..." said Sumiyoshi.
(My son... I'm very sorry. I hope one day, you'll discover who your father is. But for now, just sleep tight in Tamayo's arms...) thought Yoriichi. (Just know this, my son: your father loves you. He always will.)
In that moment, the boy named Goku Tsugikuni woke up.
Chapter 29 - End