Demon Slayer: Kimetsu no Yaiba: Hinokami Flames

Prelude: Muzan Kibutsuji

Everyone always has a backstory, especially the antagonist. Almost a thousand years ago in the Heian Era, there was a baby whose heart stopped multiple times, while it was still in its mother's womb. And by the time it entered the world, it was considered a stillborn.

"This is ludicrous!"

"Its heart wasn't beating, and it couldn't draw a single breath. If anything, it's not even a living creature."

"We're talking about killing a child that hasn't even experienced life!"

"And if we keep a lifeless child around, word will spread out and the people will wipe us out. My decision is final! It shall be cremated tomorrow!"

The next day, the baby was about to be cremated and the entire household watched. A number were against it, while most stood in silence. But just before the baby could be burned to death, they heard a sound that shocked everyone. The baby was crying. It was alive, giving it a chance at life.

But nothing good comes without a price. As the child Muzan grew, he was suffering an illness that led to Thanatophobia. He then learned that he was diagnosed with a disease that would kill him before he reached 20. He would then have a doctor that did everything he could to help him with his treatment.

"Here. This medicine will help with your treatment."

Muzan drank the elixir and waited patiently for the outcome. But as he grew patient, his condition worsened; causing Muzan to resent all living things. But then, one day as the doctor take notes, Muzan walks outside, grabs a knife and stabs him in the head. "May your incompetence lead you to your death. Get that failure of a man out of my sights!"

However, as the days kept going by, Muzan reached 20. "So, it did work after all." Muzan's frail and weak body had become energetic and stronger. On top of that, he can manipulate it at pure will. Soon, he realizes one day that his survival instincts tell him that stepping outside will kill him.

He tested the waters and pointed a finger out. When it made contact with the sunlight, it felt like it was in hell burning to ash. "I'm cured. But I can't make contact with the sunlight. Dammit!" After a full day, even after eating three meals, Muzan began to feel cravings for human flesh.

In the vast darkness of the night, he started to devour humans. It wasn't long until the people started to realize Muzan was the human devourer. Soon during his evening hunt, a platoon of soldiers ambushed him. "Muzan Kibutsuji, we're aware of you eating people like the animal that you are! You have sentenced to death!"

"A bunch of meaningless humans, judging how my life should end? Now that's hilarious. If you run away now, I'll forget this embarrassing transgression."

"Never! We know what you are, you demon!"

The soldiers fire their arrows at Muzan who doesn't appear fazed. "A demon huh?" He swats away the arrows and slaughters the men killing them instantly. "What a way to describe me." But despite his immortal power, he still was forced to hide from the sunlight. "This is such a humiliation!"

So, to find a way to resist the sunlight, Muzan started to examine the doctor's medicine. "So, the doctor only gave me and experimental medicine. Because of a missing component, it couldn't be complete. The missing component is called, the Blue Spider Lily." One of the rarest flowers in all of Japan, and it was known by a number of people. And the only one who knew much about it was the one person who Muzan killed out of spite.

So, now in order to find it, Muzan began turning people into demons hoping to find his miracle. They soon spread like wildfire across the country. Eventually, he met Michikatsu Tsugikuni, a demon slayer, and manipulate him into thinking that the mark would kill him at the age of 27. As he feared death, Michikatsu took up Muzan's deal into becoming a demon.

Muzan then encountered an ill Tamayo, who wished to see her children grow up and allowed him to turn her into a demon. However, her nature was very feral, and she devoured her entire family. She resented Muzan for turning her into a violent monster but could not fight against him. She became enslaved to him as a servant.

Then when the two wandered around looking for information, he came across a man similar to Michikatsu, his younger brother Yoriichi Tsugikuni. "Hmph! Another ammeter swordsman? You're all just the same." And that was his mistake. He underestimated Yoriichi and his devastating strength. And through massive swings of his blade, he strikes fear into the demon king's hearts, and was very injured.

"Tell me, what is the value of a life to you?" Yoriichi asked. "You've killed so many because of your existence. You have become a disease. So, what is life in your eyes?" Muzan refused to answer him and bursts his body into 1,800 small pieces with 300 escaping.

Muzan has never felt such humiliation in his life. "Grah! A human that doesn't fear me! This is not possible!"

"I assumed that you faced my younger brother." Kokushibo said.

"That... monster is your brother?!"

"He is. He is the reason that the demon slayers have grown in new power. These breathing styles have become more effective against our kind, especially the Sun Breathing techniques."

When Yoriichi died, Muzan and Kokushibo slaughtered every Sun Breather ensuring that another Yoriichi could never be born. But Muzan still feared him even after he died. So, he began to have all the lower demons fight the demon slayer corps.

Then to counter the Hashira, Muzan then formed the Twelve Kizuki, which consisted of Kokushibo, a martial artist fighter Akaza, a cult leader Doma, who recruited a pair of siblings, Gyutaro and Daki, a mischief criminal Hantengu, a dying killer Gyokko, a murderer who enjoys death Enmu, and a frail sick boy Rui.

Soon in the Taisho Era, Muzan would come across information.

"Descendants of Sun Breathing?!" Kokushibo asked shocked.

"Correct." Muzan said. "They were believed to be hiding in the mountains as coal sellers."

"Coal sellers? The only ones that sell coal back then were... So, he passed is onto that bloodline."

"Excuse me?"

"Yoriichi passes his Sun Breathing Style to the Kamados. He saved them before I became a demon."

"Well then, suppose I'll have to pay these Kamados a visit."

"If I may Lord Muzan, allow me to destroy these sun remanences. It should be my responsibility to destroy my brother's legacy."

"*sigh* Very well. You shall go in my place. But you are to see if any of them can withstand the sunlight. If they still resist, do not hesitate to destroy them. Do not fail me."

"Of course."

Now after so long, Muzan has found the answer to his salvation. After a millennium of setbacks, he shall gain his immortality in the final battle.