After Muzan left the Kibutsuji Residence, he began his journey with several goals in mind: to find the other Blue Spider Lily, to discover a way to conquer the sun, to become stronger without killing or eating humans, and to find loyal subordinates.
Muzan knew that the Blue Spider Lily would only bloom in the daylight for a few days, once a year.
50th Year of the Journey.
After finding the stem, Muzan creates a wall and a roof every night and left out some space in the center so the sunlight can get through it during the day.
He knew that he is vulnerable in the sunlight but he still have some resistance.
After getting burned a little by the sunlight,
Muzan had finally got one Blue Spider Lily. Muzan then experiments with it, he still couldn't understand how it had turned him into a demon. Instead of discarding it, he used it with a newly created technique called Devour, an ability that allows him to absorb or store something inside his body.
Armed with memories from his past life, Muzan tried using Sun Breathing and the See-Through World, but he failed immediately. It was as if the laws of the world themselves restricted him from using these techniques.
101st Year of the Journey.
Muzan discovered a mysterious book and opened it. Upon doing so, he inherited someone's memories.
"A war erupted in the heavens. The Demon God Satan had begun a war to kill every god in existence-to devour them all and become an even more powerful being.
In response to his evil deeds, I, the Ruler of the Gods, awakened from my slumber. With my katana at my hip, I introduced myself as the Sun God, Amaterasu.
Yet before I could even finish speaking, the Demon God attacked me. I soon realized how slow he was. In a single instant, I used my strongest ability, and the Demon God burned to ashes.
The Demon King then attempted to attack, but he was even slower. I sliced him into hundred millions of pieces in the blink of an eye.
The war ended in their defeat. The remaining gods celebrated our victory, but I could not. I realized then that the Demon God had predicted everything.
He had done something before the war even began. The Demon God and Demon King had already sent their souls to the lower world, searching for suitable vessels, and left their memories in them.
What I had killed were gods with no souls-bodies moving purely on instinct. That is why they never responded to me.
I sensed their souls… and also a blue flower filled with demonic blood in the lower world—something that would greatly aid in their rebirth as demons.
I could not simply sit and watch. So I decided to reincarnate.
Retaining my full memory and godly powers would be too difficult. So I traded them away in exchange for my breathing technique and a few useful abilities.
I don't know which era I will reincarnate into, but once I begin the process… I may lose all my memories."
Muzan's face broke into a cold sweat.
"That memory… it belonged to the Sun God. If he truly reincarnated… then his reincarnation must be Yoriichi Tsugikuni."
"But the Demon God Satan and Demon King, huh? Who are they? The Demon God may be familiar as I inherited some of his memories. But his memories of my is only about his ability, Devour, and how he ruled as the Demon God. Maybe the blurry right-hand man in Satan's memories is the Demon King this Sun God Amaterasu's talking about."
202nd Year of the Journey.
Muzan had traveled to every country on Earth.
But before continuing his travels, he tested a theory: he placed the Blue Spider Lily stem under direct sunlight. After a hundred years, its color changed-from a pale bluish hue to a brilliant golden color. It was now what he called the Golden Spider Lily.
After much contemplation, Muzan ate it.
It tasted the same but just sweeter. At first, nothing seemed to happen. But deep within his body, his blood began to change. Once violent and chaotic, his blood now circulated normally, like that of a human. Its color shifted too, from crimson-red to a radiant golden-red.
On a beautiful sunrise, Muzan stood atop a mountain.
He didn't burn.
He's not vulnerable to sunlight anymore.
He simply stood there, tears streaming from his eyes.
"I can finally walk in the sun again!" he cried and instantly vanished.
But even after conquering the sun, Muzan realized something: he was still weak.
Even with the ability to move a quarter kilometer per second, he found himself lacking, especially after witnessing the memory of the Sun God, who had defeated the Demon King in the blink of an eye.
So Muzan hid away, dedicating himself to training. He tried to recreate the abilities of main characters from the anime and manga he liked. First, he attempted to use chakra but failed. As chakra didn't exist in this world. Then, he tried mana, but that failed as well. This world only had Breathing Techniques and Blood Demon Arts.
Thus, Muzan created his own new abilities.
He forged new Blood Demon Techniques and developed a state he called Nirvana.
In this state, his mind became so calm that no mental attack could affect him. His perception of time slowed-though in reality, it didn't. His thoughts simply accelerated a thousandfold.
That meant, in his perception, time moved a thousand times slower-making him move a thousand times faster than before.
303rd Year of the Journey.
Muzan had finally mastered all of his abilities. At last, he chose to return to the outside world.
A few months later…
In a certain evening.
"I've finally found you, brother," a young man in his early twenties said, his voice trembling with excitement.