[Demon Slayer] Reborn as Muzan, How Do I Become an Upper Rank Demon?

[A companion piece to the Naruto story next door.]

After transmigrating, Utsu Kueguang became a person of the Heian era.

But this world was different from his previous life. Here, he had parents who loved him, an illustrious surname, and... an older brother who was frail and sickly and didn't particularly like him: Fujiwara Tsukihiko.

Although his older brother didn't like him, he truly loved him. On the first day after he transmigrated, the first thing he saw when he opened his eyes was this child, with black curly hair and crimson eyes, sleeping beside him.

But his brother had been on the brink of death since birth. His family had close ties with a clan of priests, and a physician arrived at the Fujiwara home with a prescription requiring the Blue Spider Lily... More and more clues made him suspect that his black-haired, red-eyed, all-too-familiar older brother was none other than Muzan Kibutsuji, the villainous boss from the anime Demon Slayer that he'd watched in his previous life, the one whose every appearance was met with a barrage of comments about sunlight and Wisteria.

But that didn't matter. He could bear all these sins on his brother's behalf.

Fujiwara Kueguang, no, Muzan Kueguang thought, and followed in his brother's footsteps.

Please note:

1. Two protagonists, no romance between them. The brothers are Muzan Kibutsuji and his younger brother, Kibutsuji Kueguang. There will be portrayals of the Kokushibo and Kaigaku pairing, as well as the Obanai and Mitsuri pairing.

2. Kueguang's child body affects his mind. His experiences with aristocratic education in the Heian period after transmigrating will change some of his personality and values. In the Demon Slayer world, he's a villain who revolves entirely around Muzan, though he's a normal person when it comes to anything unrelated to his brother. In the Naruto companion piece, he's a genuinely good person.

2. My writing isn't great, and the characters may be out of character. I'm writing this for love, so please don't flame me. Updates are whenever I feel like it.

3. The original plot may change drastically. The demons win, but simply ignore the Demon Slayer Corps and go abroad in this alternate scenario. I honestly can't think of how the demons could lose without being nerfed. There may be bonus chapters crossing over with other anime.

4. I like both the Demon Slayer Corps and the demons, and I'm an utterly devoted fan of Kaigaku, Muzan, Tanjiro, and Yoriichi, so I'll try to be fair and write developments that make sense.

5. The protagonist is overpowered, and is a swordsmanship genius. But in the world of Demon Slayer, facing Yoriichi Tsugikuni, the best he can do is hold him off for a while before escaping.

6. As someone who transmigrated, Kueguang will make snarky comments and references in his thoughts. However, he's only watched the Demon Slayer anime, and stopped at the Entertainment District arc, so he doesn't know where the Blue Spider Lily is.

7. In my headcanon, the Ubuyashiki Family used to bear the great Heian-era surname Fujiwara. Muzan's human name was Tsukihiko. After Muzan became a demon, the Fujiwara branch he belonged to was forced by the main family to change its name to Ubuyashiki and bear the entire curse. The protagonist is Muzan's ultimate fan, and Muzan's wishes are his highest priority. He'll also take good care of Kaigaku, whom Kokushibo picked up, because he thinks Kaigaku is cute and catlike.

---- I no longer have the energy to write after starting work, and I've also left the Demon Slayer fandom. There's a good chance I won't be able to continue this story. I've put the excerpts I wrote before below the blurb. Thanks to everyone who bookmarked this story! ----

Written on September 15, 2024

"...You reek of sweat. What on earth have you been doing?" Fujiwara Tsukihiko furrowed his brow, but before he could finish what he was about to say, he started coughing repeatedly. He only gradually calmed down after Fujiwara Kueguang, flustered, hastily wiped his hands on his robe, drew him into his arms, and gently patted his back.

Tsukihiko smelled the clean scent of soap from his younger brother's clothes, but the sweat mixed in with it was almost unbearable. He managed to raise a hand, cover his nose and mouth with his sleeve, and take a few breaths before ordering Kueguang haughtily, "Let go of me. Don't get your sweat on my clothes."

His complexion was terrible. His skin was far too pale, set against his scarlet eyes and the dark purple lips that came from years of illness. Sharp canine teeth were just barely visible behind his lips. These features, combined with his menacing expression as he lurked in the shadows, nearly made the woman standing nearby lose her grip on the tray.

Her body trembled faintly, and even her toes instinctively pointed toward the door. She didn't know which was more terrifying: the demons of legend or young Master Tsukihiko, lying in bed with little time left to live yet still holding the lives of all the servants in his hands. But she knew that whenever young Master Tsukihiko had a fit of coughing like this, he would lose his temper. Only his younger brother, young Master Kueguang, was spared.

Fujiwara Kueguang glanced at the maid without making it obvious, then waved for her to leave. After she escaped, visibly relieved, he took the medicine prepared by the physician from the food box behind him. He put one arm around his brother's back and carefully helped him drink it.

"I was practicing swordsmanship."

As Tsukihiko sipped the medicine, he heard his healthy younger brother's reply.

His face contorted for an instant, but in the end, he said nothing. He only swallowed the bitter medicine faster, humiliated, while his fingers beneath the covers nearly pierced his own palms.

Why? Why?

Fujiwara Tsukihiko was going mad inside. Jealousy made him forget the smell beneath his nose. He had nearly stepped into the underworld at birth, and even now he was merely clinging to life. His parents had always provided for him, but he knew his family and even the servants cared for him only because he looked as though he had little time left, and wouldn't be a burden for many more years.

His younger brother, born barely a year after him because their parents couldn't wait to have another child, was the healthy and clever one they truly wanted as their heir.

He could only remain in the shadows, while his brother could bathe in sunlight. He could only drink bitter medicine and eat plain rice porridge, his stomach so frail he couldn't even taste pickles, while his brother could freely enjoy fine food and wine. He bore a name meaning moon, while his brother had been given the name of Amaterasu, the sun goddess of Takamagahara. His eyes were blood-red, while his brother's were clear. His hair was naturally curly, while his brother's was straight like their parents'...

Would there now be something else his brother had that he didn't? He could only stay indoors, confined to his bed, while his brother could run freely and practice swordsmanship.

The endless flames of jealousy nearly filled Tsukihiko with murderous intent. But just as he was about to be buried under those negative emotions, Fujiwara Kueguang spoke again.

"Brother."

Fujiwara Tsukihiko watched as Kueguang set down the medicine bowl, gently laid him back on the bed, and then clasped his hands between his own. The warmth made Tsukihiko feel as if he could sense the heat of sunlight.

"I will be your blade, your sword."

Fujiwara Kueguang made this vow.

He watched his brother's expression change unpredictably. After a moment's hesitation, he decided to say everything on his mind. After all, he and Tsukihiko weren't the pair of twins from the later Tsugikuni family. He would respect his older brother more than Yoriichi did, until Izanami took him into her embrace.

"I know you don't like me, Brother, because I'm healthy." A hint of sadness crossed Fujiwara Kueguang's face. He opened his eyes, which he rarely did after Tsukihiko had once scolded him over their color. Their similarly crimson eyes met. "So please make me a part of you, Brother, your flesh and blood... I only study swordsmanship to protect you, Tsukihiko."

He addressed his brother by name, rather disrespectfully. Then, as the other fell silent, growing even more resentful and perhaps thinking he was showing off, Kueguang continued, "I will make you head of the family." Fujiwara Kueguang spoke with absolute conviction. "And you can use me however you want. Even if you want me to become an assassin..."

Fujiwara Tsukihiko stared at him in shock. He hurriedly interrupted his younger brother, disbelief filling his blood-red eyes. "You... Do you really mean that, K... Kueguang?" His voice was strained, as though saying the name aloud would wound him.

Even in his jealousy, even after murderous thoughts had taken root, he had never considered letting his own younger brother become a mere assassin, the dogs nobles used to eliminate their rivals.

Tsukihiko looked at Kueguang, who had lowered his gaze beneath his thick lashes once more, and felt conflicted. He struggled to sit up and reached out to touch his younger brother's cheek. "...I want to live."

"I know, Brother. You want to live." Fujiwara Kueguang's expression was calm. He clenched both fists and rested them on his knees as he knelt upright. "Father asked the Imperial Family and the main branch to find the best physician. The physician said he would do everything he could to preserve your life. The medicine you drank today is his new prescription."

Then Kueguang leaned forward and spoke in a whisper to Fujiwara Tsukihiko, whose body began trembling after he had managed to sit up for only a few minutes. "If that still can't cure your illness, I'll ask the priests to give you half of my lifespan."

Fujiwara Tsukihiko was like a drowning traveler who had just grasped a lifeline. He stared intently at Fujiwara Kueguang, whose expression remained composed and whose smile had not faded, while his chest heaved violently. "You mean it? Really...?"

Fujiwara Kueguang didn't answer. He simply raised his arm and let his sleeve slide down, revealing his smoothly muscled forearm.

A hint of displeasure flashed through Tsukihiko's eyes, but it quickly gave way to joy. There, surrounded by Wisteria, was the mark required for the priest clan's secret art. His younger brother hadn't lied. He truly wanted to share his life with him. Only those who willingly gave another person part of their lifespan could receive this Wisteria mark, and only those bearing it could share their lifespan with someone else.

Fujiwara... Tsukihiko...

Fujiwara Tsukihiko stared, utterly satisfied, at his own name in the center of the Wisteria on his younger brother's arm. He felt happier than ever, so happy he could almost laugh.

"Then let me try that new physician's medicine again, Kueguang." His breathing settled, and he resumed his old haughty manner as he gave his younger brother an order. But there was far less urgency and agitation in his voice now, as though Kueguang's actions had given him confidence.

"All right." A tender look shone from Kueguang's eyes, barely open. He returned the medicine bowl to the food box, took out a bowl of plain rice porridge, and patiently fed all of it to Tsukihiko. Only after carefully wiping his brother's lips and tucking in the covers did he quietly take his leave. "Then I'll be going now. Please get some good rest, Brother."

He walked out of the room into the sunlight. This time, what clung to his back wasn't his brother's resentment, but a gaze filled with hope.

At last... I can do something for my brother, too.

Fujiwara Kueguang looked contentedly at the mark on his forearm and gave an almost inaudible chuckle. He lowered his sleeve, then gently reminded the maid, whose face was blushing as she looked at his striking features, so like his brother's yet more masculine, to take good care of Tsukihiko. When she clutched her sleeve and hurriedly promised to do so, he raised an approving eyebrow and headed for the front courtyard with the food box.

After saying goodbye to his brother, he still had plenty more to do.

Story theme: An explosive display of my Muzan fanboy side.