"Shall we retreat like this?" Akaza curled his lips with excitement. The battle had been exhilarating, the Hashiras of the Demon Slayer Corps proving to be worthy opponents.

"We could have wiped out the Demon Slayers today, yet Muzan-sama relented because of that man," Doma remarked with a chuckle.

"Who is he? He's unbelievably strong, like, ten times stronger than Kokushibo." Akaza remarked.

Koku seethed, his veins pulsing. He refrained from drawing his sword against Akaza, since they had fought together today.

"As long as that man stands with us, the Demon Slayer Corps is doomed to fail." He stated.

Daki sat by the side. Having experienced a narrow escape to death, she had turned into a more serene version.

Her eyes overflowed with admiration for Muzan.

Muzan-sama has changed, willing to put herself at risk to save me.

"In that case, Kokushibo, do you know who that man is?"

Kokushibo, or rather, Michikatsu Tsugikuni, fell silent. Before he could respond, the Biwa Demon had melodiously strummed the instrument, and all the demons were transported out of the Infinity Castle.

...

So there never was a system; it was all his own strength.

Teru clenched his fists, deep in thought.

Everything flooded back.

"I am from the 26th century... well, around 600 years ahead of now, having travelled through a spacetime tunnel. I am...the sole immortal on Earth, born in the 21st century, living into the 26th century"

Teru spoke with uncertainty. Muzan caressed his healed hand, listening intently.

"Why did you come?"

"To find you, of course." Teru replied with a smile. "Eternity is torture when you don’t have companions. I don't fear sunlight; to me, immortality is absolute; even the black holes can’t kill me. However, unlike demons, I can't transform others into beings like me. In the future, despite human lifespans increasing, science has shown limits, and it is around 250 years. Over centuries, I altered my appearance, manipulated behind the scenes, seized the world's greatest power, indulged in every conceivable pleasure, only to find it all insipid. I’m too lonely. By chance, in a highly credible history book, I noticed your existence, another immortal. So, I rallied the world's most advanced research forces, dedicating 60 years to create a stable time-travel machine. I merged it with my undying soul... well, souls do exist, which was testified in the 23th century...to traverse here and find you."

The author's tale has been misappropriated; report any instances of this story on Amazon.

Muzan let out a soft chuckle.

"So you've orchestrated our meeting long before. I thought you were genuinely just an ordinary person, the heir of a medical lineage. I even strived to keep you unaware of demons."

"Only a moment ago did I recall my true identity. Before then, my memories were constrained by something... perhaps, the will of the cosmos, rejecting an entity not truly belonging to this era? Well... when I return to my time, I'll initiate research on that matter. Oh, and I've brought the blue spider lily with me; you should consume it now."

The sun was rising. After ingesting the flower, Muzan opened the window, extending a cautious finger into the sunlight.

She managed to feel no pain. No burning sensation. Her millennia-long aspiration had come true.

Teru opened the door and window, gently pushing Muzan outside. She basked freely in the warmth of the morning sun, experiencing a sensation she hadn't felt in almost a millennium. Teru affectionately watched her, taking out a massive black umbrella.

"So this umbrella is useless now."

"Where did it come from? I should have asked you before."

Teru sensed the spacetime tunnel within his soul, and he told Muzan that once he thought he had a ‘System’.

"That was another restriction of spacetime. The fact is, both power and items are my doing. I bound a spatial device to my soul, housing many things you might need, like the black umbrella and the blue spider lily."

Muzan happily embraced Teru's arm, holding it against her chest.

"So, what are you planning to do now?"

"That depends on what you want. I can take you back to the future directly. Humans are now exploring the cosmos; we've established space elevators, invented spacecraft of half light speed, and developed many advanced technologies you may find amusing. Or, we could spend some more time in this era."

The demon queen held her words. After a long while, she spoke:

"Can we go back to the past?"

"What?"

"I've caused too many tragedies. Many people lived a miserable life because of me. But this is wrong. Only after meeting you did I realize I might have been wrong for a thousand years. In truth, I've always regretted why I couldn't control myself, why I killed the doctor who gave me the antidote—not because it denied me the blue spider lily, but I shouldn't have treated someone who genuinely wanted to cure me that way. If it's possible, please go back to the past and stop me then, can you?"

Teru smiled happily, but bitterly as well.

If time were a river, flowing downstream and upstream were profoundly different. Opting for the future, he wouldn't lose his strength and memories; he'd recover quickly. Setting out for the distant past would make him face more resistance, possibly he would have to go through memory loss again, which was agonizing since he hated losing control of himself.

Yet Muzan was right. Even if he forcibly took her back, with her awakened conscience, she'd only be melancholic. Indeed, he was genuinely happy she said those words.

"Alright then. See you in the past."

Muzan watched Teru's form grow ethereal, vanishing into thin air.

...

Due to inherent frailty, the young woman, Muzan Ubuyashiki, barely twenty, already bore white hair.

She was standing in the middle of the courtyard, which was bathed in sunlight, but she remained untouched by it. She couldn't feel the sun's warmth.

The medicine didn’t take any effects. Why was it so difficult to hold onto her simple wish to live?

She removed her hairpin, her hands trembling violently. She needed to vent, to make others feel a fraction of her pain, hoping to find some solace.

But a warm, masculine hand clasped her delicate wrist.

At that moment of touch, Muzan recalled everything.

Teru Watanabe's form transformed from illusory to real, then back to illusory again.

The world trembled silently around them, only to settle back into tranquility.

From this moment onward, history was rewritten.

From then on, demons would cease to exist.

From then on, no one would suffer tragedies caused by demons.

And she spoke.

"Welcome back."