Prologue Part 1: The Second Corps Formation

Yoriichi Tsugikini, the legendary Demon Slayer who had the Sun Breathing forms... was defeated by Muzan Kibitsuji, and blood of the demon lord fell on one of his open wounds.

Yoriichi could feel the transformation... how his body started to change, making Muzan chuckle.

"For more that I would love to see you dead, seeing you serving me sounds way more amusing." Muzan said as Yoriichi stood up, veins popping all over his body as he shook "Now, bow before your new master."

"Enbu. (Dance)" Muzan body was cut in a vertical slash, as Muzan spit out blood.

"H-How? How did you resist me!?" He demanded as Yoriichi panted, his skin turning white as the moon, his crimson eyes now where slits, and his demon slayer mark remained.

"Does it matter?" Yoriichi asked back "I am not sure myself. But consider it my fate, to kill you." he said and got in position to keep fighting.

Muzan took a step back, as he shook in fear as Yoriichi took a deep breath "W-Wait! If you kill me now, you will die too!" Muzan pleaded "Don't you want to be immortal!? To live forever and perfect your skills more and-"

"To die is part of life." Yoriichi interrupted as Muzan eyes widen more in fear "And if I will die after killing you... so be it."

"WAIT!"

"Ju San no Kata. (Thirteen Form)" Yoriichi blade was caught in blaze as he rushed to Muzan and performed all of his breathing forms, hitting each one on Muzan.

6 hearts and 4 brains were hit and right before Yoriichi could hit his last two hits, the Sun Breathing turned on him, burning him from inside.

As Muzan saw this, he decided to escape, punching the ground creating a smoke wall and fleeing. Meanwhile, Yoriichi held his side in pain. A burning pain he never felt before. He managed to stand up as the sun started to rise.

A part of him, told him to stay there, to be burned by the sun to stop being a demon. But... Muzan is still free...

His duty is far from over.

He turned around and walked towards the forest, ready to leave the Demon Slayer Corps, but still eliminating those monsters.

*60 years later*

Yoriichi had wander the land for so long, and still no trace of Muzan. Most demons when seeing him, or most precisely his hanafuda earrings identifying him, throw themselves at him, trying to kill him under Muzan's orders from what he gathered.

He stopped seeing a curious sight. It was a demon; he could smell him as that. But the difference it was that it was demon teen chopping wood. Weird.

He stepped closer and the demon boy seemed to notice him, stopping his tracks and his eyes widened "A demon?" He gulped "Do-Do you need something?"

Yoriichi raised a brow, before he shook his head and smiled kindly "No, I was just passing by, and I got curious."

"Curious?"

Yoriichi nodded "You seem perspective." He tugged on his hanafuda earrings, making Tanjiro eyes widen. Yoriichi expected for him to attack him, but...

"You are the demon who challenged that man." The demon said, a glitter of hope in his eyes. "Tamayo-san talked about you."

Tamayo... another name he heard from demons he had slain, and even from Muzan... it was that woman who served him before their battle...

Yoriichi nodded "I guess that I am." he smiled kindly at him "Do you mind guiding me to her? I need to talk with her."

The demon nodded, smiling at him "Of course! Tamayo-San wanted to talk with you too for a while now. Oh! My name is Tanjiro by the way. Tanjiro Kamado."

"A pleasure Tanjiro, my name is Yoriichi."

*Tamayo's Home*

Yoriichi was found surprised meeting the rest of the demons in the household. First, the Kamado siblings, Tanjiro and his little sister Nezuko. Tanjiro was too kind, and according to him, neither him or his sister had eaten a human since their transformation, which in itself was hard to believe.

Nezuko seemed to doubt him at first, but if her brother believed him, she would too, showing her kindness at him.

Then is Yushiro, a demon turned by choice by Tamayo to cure his sickness. After insulting Tanjiro on how he could bring a demon here, and being scolded by Tamayo, he simply frowned and kept an eye on him.

And lastly, Tamayo herself, beautiful since the last time he saw her next to Muzan, but only this time, with her head raised.

She explained how she is trying to make a serum, capable of turning demon back into humans for two reasons; To give Tanjiro and Nezuko a normal life;

"I remember clearly when I met those two." Tamayo explained with a calm smile "Tanjiro carried Nezuko, both covered in blood and once they spotted me... he started to apologize and calling me 'Mother', his memories probably still messed due to the transformation. I want to give them back what they should have never lost."

And the other reason...

"Fight Muzan?" Yoriichi asked and Tamayo nodded, becoming serious.

"I have a theory that giving the serum to that man, will weaken him more, slowing his regeneration and maybe even killing him." She then turned to him "How can you say his name?"

Yoriichi hummed "I don't know. It is clear that I am not like other demons, maybe something to do with my blood?"

Tamayo hummed "May I ran some experiments with it?"

Yoriichi nodded as he looked Tanjiro and Nezuko talk in front of the fire place.

*Weeks Later*

"Yoriichi-San." Yoriichi stopped his practice hearing Tanjiro.

"Yes?"

Tanjiro bowed his head "Please train me!" He begged "I... I want to help in anything I can! And I want to protect people from demons and from Muzan... so please..."

"Help me become a Demon Slayer!"

Yoriichi blinked surprised and nodded "Very well. I'll make you my Tsuguko. But I don't know if you can master the Sun Breathing Form."

Tanjiro smiled at him "Well, I can create a new one, right?"

Yoriichi smiled "I guess you can."

*40 Years Later*

A century had passed since he became a demon. And he... doesn't know how to feel.

He had never eaten a human, he still trains to perfect his breathing style, and he keeps slaying demons. But... he also found people he cared for, and that they cared for him.

Tanjiro developed his own breathing style, Eclipse Breathing Form, Nezuko learned the Wind Breathing Form from the Pillar of his own generation, and Yushiro perfected his Demon Blood Art.

Tamayo is still working on a serum, but with little success.

And an idea came to his head a couple of years ago; to make a Demon Slayer Corps, comformed by demons against Muzan. If these four already are against his will, the chances of more rebel demons exist.

He took a deep breath and stood up, after meditating he made his decision.

The Moon Slayer Corps will become a reality.

(A/N: The name 'Moon Slayer Corps' is a holder. Any better sugestion will be accepted.)

Next: Prologue Part 2: Recruits