The Good Demon [KNY]

Alternate History: Fear The Red Comet

"A dance of the fire god, you say?" Rengoku raised an eyebrow.

"Y-Yes, sir.  Do you know any breathing style like that?" Tanjiro asked timidly.

Tanjiro, Inosuke, and Zenitsu had boarded the Infinity train and met with Rengoku.

...

...

...

"There is only one thing that reminds me of such a thing." Rengoku raised a single finger.

"Oh really? Could you tell me-"

"But it is not a fire god..." Rengoku's voice lowered.

"...Huh?"

"But a fire demon." Rengoku said. "Centuries ago, before the Demon Slayers were even an idea, an entity appeared." Rengoku began.

"Whenever demons prowled, and whenever man rose up to fight against them, it appeared. In the midst of battles, or nearing their conclusion, anywhere conflict between man and demon flared, the red flame appeared into the sky." Rengoku said. "The Red Comet." Tanjiro shuddered at the way Rengoku said it. "The Red Comet would come crashing down, and begin its onslaught." 

"It aided other demons?" Tanjiro asked.

"No. Heavens no. It killed the demons. Brutally." Rengoku chuckled, and Tanjiro was even more confused.

"So it helped people...?" Tanjiro was trying to understand this thing's motives.

"According to myth, the Red Comet coats itself in flame, before doing battle with its own kin." Rengoku ignored Tanjiro and continued. "After decimating its demon opponent, it turns its attention to the human. That's when the most intriguing thing happens..." Rengoku trailed off.

"Please continue!" Tanjiro barked suddenly.

"Of course!" Rengoku smiled, before immediately getting serious again. "No one knows how, but the Red Comet heals the fighter that opposed the demon, before challenging them as well. The human always loses, of course." Rengoku sighed at that. "Despite the Red Comet's terrifying power, it has never once killed a man." Rengoku muttered. "Some say that when it first rose, it murdered an entire city, and its bloodlust was forever sated then. Others say it is a genuine ally to man, but fears joining them. Some say it's a reincarnation of the first demon slayer, a raging spirit that seeks to erase demonkind and test the strength of the current generation." He gave a few of the many rumors behind the origin of this 'Red Comet'. 

"The second one is the closest." 

Tanjiro and Rengoku turned around suddenly.

An... unusually large boy sat on the seats behind them. He held his head low, staring at the ground. They could not help but notice the Comet-shaped scar on his cheek.

"Truth is, he just fucking hates him." The boy muttered, referencing the progenitor of demons. "He goes around killing demons and doing battle with demon slayers, honing his strength. He's been doing this for the last thousand years..." He chuckled at that, though they didn't know why. "He was wronged by the first demon and swore revenge. So until the day he shows himself, the Red Comet will prowl these mountains. Hindering the spread of demons and growing stronger." He finished.

"...I'm sorry, but who are you?" Tanjiro cut in.

"Oh, me?" The boy asked.

"I'm Akai Suisei."

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The demon slayers would soon sniff out Enmu, and defeat him. Only to be met with Upper Moon 3 soon after.

On that night, Kyojuro Rengoku faced Akaza.

On that night, the Red Comet came crashing down.

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This is an alternate history where Akai wasn't an idiot and didn't chase Muzan a few days after becoming a demon. This Akai has lived and experienced a thousand years of life, all spent towards acquiring the strength to one day face down Muzan. In this alternate timeline, Akai had been the one to hunt down all lower moons, only missing Rui. He then moved on to the Upper Moons. He would go on to appear in the demon slayers' battles with the Upper Moons, sometimes letting things play out, but always making sure that in the end, the demons were the ones that were dead.

Sadly, this Akai never attained his goal. He was sucked into the infinity fortress and did battle with Upper Moons One, Two, and a revived Upper Moon Three. He was defeated, and was never seen again.