One thousand years ago, Muzan Kibutsuji was born as a stillborn infant. Only through a primal, desperate will to live did he manage to survive. However, due to his frail constitution, he was diagnosed with a terminal illness—he wasn't expected to live past the age of twenty.
He eventually met a kind-hearted doctor who developed a special medicine to save his life. When the initial results showed no improvement, Muzan, in a fit of rage, murdered the doctor. Shortly after, the medicine took effect; his body became perfect, granting him god-like strength.
The only defect: he could no longer walk in the sun.
Refusing to accept a weakness in his "godly" self, he spent a millennium obsessed with the doctor's notes. He discovered the key ingredient was a mythical plant called the Blue Spider Lily. For over a thousand years, he created legions of demons to find it, but he never did.
Now, he would never have the chance to become the perfect being. He was going to die at the hands of this terrifying boy. He was unwilling—beyond unwilling!
"KAMEHAMEHA!"
Goku didn't care about Muzan's regrets. He thrust his palms forward, and the blue pillar of energy completely engulfed the paralyzed Demon King. When the light faded, Muzan Kibutsuji had been utterly erased from the world.
In that same instant, every demon on the planet—except for Nezuko, Tamayo, and Yushiro, who had long since broken Muzan's curse—simply dissolved into ash.
This was the sight that greeted the nine Hashira and the army of slayers as they arrived. When the seven Upper Moons had been whisked away by Nakime's Blood Demon Art, the Hashira had immediately finished off the remaining lesser demons and spent hours searching the Infinity Castle for Goku.
They arrived just in time to see the final Kamehameha send the progenitor of all evil to hell.
"Muzan Kibutsuji... is he really dead?" the Hashira asked, their voices trembling with disbelief.
Goku nodded and handed several crystal vials to Tanjiro. "Here. I collected the blood from the seven Upper Moons and Muzan. I hope this helps."
"Thank you... Thank you so much, Goku-san!" Tanjiro took the eight vials, his eyes brimming with tears. With these samples, Lady Tamayo could finally finish the medicine to turn Nezuko back into a human.
The rest of the slayers looked at Goku with profound awe. They realized that while they had struggled against the Upper Moons, Goku had taken them all on—plus their King—and won single-handedly. His strength didn't just rival the nine Hashira; it eclipsed them entirely.
Tears of joy broke out across the battlefield. For the first time in a thousand years, the Demon Slayer Corps had won. The war was over. No more children would be orphaned; no more families would be devoured. And it was all thanks to the boy named Son Goku.
...
One Month Later: The Headquarters
"No amount of thanks could ever be enough," Kagaya Ubuyashiki said. His face was clear, the purple curse that had plagued his bloodline for a millennium having vanished the moment Muzan died.
"This is a small token of my gratitude. I hope it helps you on your path." He handed Goku a leather-bound book titled 'Records of the Demon Slayer Marks, Bright Red Blades, and the Transparent World'.
With the demons gone, Kagaya had officially disbanded the Corps, providing for every swordsman so they could live peaceful, normal lives. During the past month, the Hashira had kept their promise, sparring with Goku and teaching him their respective Breathing Styles.
Every elite slayer had contributed, including Tanjiro, who taught him the most powerful of all: the Sun Breathing.
By synthesizing all these styles, Goku had perfected his own Breath of Ki, creating ten distinct forms. Recognizing Goku's passion for martial arts, Kagaya had compiled all the secret lore of the swordsmen into this manual.
"Thanks! I'm going to head out now to continue my training. I'll come back and visit you guys when I can!"
Goku tucked the book away, waved goodbye to Kagaya, and leaned against the wall. He closed his eyes and vanished.
Watching the empty space, Kagaya sighed in wonder. "You must truly be a god sent by the heavens to save us."
...
The White Space
In the familiar white void, Goku stood before the fourth door. This one bore the inscription: Fairy Tail.
With less than two years until the next World Martial Arts Tournament on Papaya Island, Goku knew he needed to push himself even harder if he wanted to win the championship. He pushed open the door and stepped through.
He materialized in a quiet alleyway in Magnolia Town, within the Kingdom of Fiore on the continent of Ishgar.
"I should see if there are any strong guys around to spar with."
Goku stepped out of the alley and into a vibrant, bustling street. The architecture was different, the clothes were colorful, and the atmosphere was lively. Having traveled from Mount Paozu, Goku was used to strange things—flying metal birds, fast boxes, and capsules that turned into houses—so the novelty of the scenery didn't distract him.
He walked up to a group of men sitting at a table playing cards. "Excuse me, mister! Do you know who the strongest person around here is?"
His mission was always the same: find the best, learn their secrets, and keep getting stronger.