Tanjiro and the unconscious bodies of Inosuke and Zenitsu were sent flying as Nezuko planted a flaming kick straight into Akai's face. He flew back and through three dozen trees. Midair, Akai reorientated himself and planted his feet into the ground. He was panting wildly, catching his breath after having accumulated so much damage that day.
Nezuko stared him down with glowing eyes. She had nearly doubled in size and become much more muscular, looking like she had grown years older. All throughout said body was unusual red marks that looked like thorny rose vines. A single, blood-red horn sat on her head. On the right side of her cheek, he saw what looked to be a rose bathed in flames. The flame bent in such a way that from far away, it resembled...
A comet.
"Okay, what the hell..." Akai muttered, heating his limbs as Nezuko blasted towards him. Akai threw a red-hot fist while Nezuko threw a kick that glowed pink. The attacks collided and let out a shockwave, splintering the surrounding brush and singe-ing all of the grass around them. The area soon disappeared in a flash of pink and red as Akai and Nezuko traded blows at incomprehensible speeds.
"What the-" Akai hissed through his teeth. "She's stronger than Akaza!!!" Akai panicked as she began to overpower him.
At that moment It began to dawn on him.
Of course she was stronger than him.
She's him.
But better.
Over every one of the battles Akai has fought, his body has accumulated damage from both the attacks of other demons and himself. Despite his power output growing stronger, his body was growing weaker; breaking down over time.
He was-
Akai was cut off as Nezuko punched him in the side, cracking his ribs on impact.
"Probably should focus on staying alive." Akai noted.
Nezuko blew through Akai's defenses and planted a kick into his chest, sending him flying back. She ran forward and jumped up, grabbing his head midair and slamming him down onto the ground.
"Meshuko... wai..." Akai muttered weakly, face buried in the ground.
Nezuko let a manic smile grow on her face as she broke into a sprint, still holding Akai's face in the ground and grinding him into the forest floor. She painted the forest red with his face, a crimson line painted on the ground with Akai's blood.
Nezuko lifted his head half-erased head suddenly and held him in front of her, ramming him into another three dozen trees.
"I can't match her..." Akai thought he was blasted through another tree.
Nezuko flew through one last tree with Akai and spun, throwing him hundreds of feet into the air. Akai healed himself as Nezuko brought her hands together, slicing open her wrists.
"Oh no." Akai realized what she was doing.
Nezuko locked her eyes on Akai's falling body as the blood coiled from her wrists, coalescing into a fiery orb, growing in size as she poured more and more blood into it.
"Not my own technique..." He whimpered.
The orb grew larger and turned pink as she heated it further.
"If there is anything I will regret, it is making Aoi cry..." Akai sighed as Nezuko finished charging the sphere. She laughed maniacally as she fired the orb at Akai. The ball of death shot its way through the sky and collided with Akai. Lighting up the sky with an explosion.
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"...Something tells me Akai has made a mistake." Ubuyashiki muttered as the shockwave of the attack echoed from a few miles away.
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Akai awoke in a small crater. His entire bottom half was erased by the attack. He realized how lucky he had been. Had Nezuko perfected it, and added the golden ripple, he might have been erased entirely. He groaned and bit into his arm, slowly healing his wounds. He wasn't able to heal much as Nezuko strode into the crater and stomped on his chest. Akai could feel his reshaped sternum shatter into dozens of pieces as he vomited blood all over his chest. She stomped him over and over, eliciting screams of agony as Akai's bones were fractured and broken.
Akai, along with his bones, snapped.
"RAAAAAAAAAAAAAAAAAAAAAAHHH!!!" Akai let out a pulse of flame and heat, sending Nezuko flying out of the crater. Akai breathed raggedly as he pulled himself up, heat pouring off of his body.
"This is the third time I have been physically abused by a woman. And it will be the LAST!!!" He roared furiously. "SO HERE I-"
Akai was cut off as Nezuko rocketed forward and heated her foot to the max, planting a Red Comet straight into his crotch. Akai's face lit up in unimaginable pain as he lost consciousness on the spot. Nezuko went further, burning through Akai's body and splitting him in half vertically.
Righty and Lefty collapsed to the ground, clutching their crotches in pure agony.
"You think splitting in half would lessen the damage, but it hurts twice as much!!!" Righty thought as he wailed in pain.
Nezuko slowly began to chuckle, before laughing maniacally, charging another True Red Comet as the brothers slowly tried to crawl away. The orb grew larger, and larger, and larger.
There was no stopping her.
There was nothing that could stop that power.
There was nothing that could stop that red-hot heart beating in her chest.
...
...
...
Except itself.
Out of nowhere, the powerful, thrumming heartbeat emanating from Nezuko's chest stopped, as the glow in her chest began to wane. The blood orb charging in her hand died, fizzling out and falling into a puddle at her feet. She wailed in pain and clutched her chest, falling to her knees and shrinking to her normal size. Her body cooled and the marks disappeared, horns sizzling and shrinking away. She let out ragged, pained breaths as her heartbeat grew erratic and weak.
Her heart had given out.
...
"CAPITALIZE!!!" Lefty shouted to Righty and they rushed toward one another. They collided and fused back into Akai, who quickly brought his hands together.
"Kaaaaaa... Meee...-Wait-I mean- True... Red..." Akai corrected himself and charged a True True Red Comet. Filling an orb of blood with all of the heat he could muster and filling it with the golden ripple.
"Akai, stop!!!" Tanjiro ran in front of him and shielded Nezuko. "Remember who you're fighting!!!!"
Akai realized he was about to erase Nezuko, and depowered the blood orb, flicking it away. He himself crumpled to his knees, utterly exhausted. But he was forced into action as he saw Nezuko creep towards Tanjiro from behind his back.
"Tanjiro, move!!!" Akai shouted as Nezuko sprung to life and charged towards her brother. He shoved Tanjiro out of the way as Nezuko pounced, colliding with him and straddling him to the ground. She opened her mouth and came forward, biting down on his neck. Akai cried out in pain as she pinned him to the ground and began to drink. She let his blood pour into her mouth, her cheeks growing rosy as the delicious taste coated her tongue. She let out a muffled moan of ecstasy into his neck at such a flavor.
"Why is this oddly erotic...?" Akai choked as his body began to grow pale.
"NEZUKO, STOP!!!" Tanjiro shot up from the ground and ran towards them. "AND AKAI, STOP LOOKING LIKE YOU'RE ENJOYING IT!!!" He glared at the Red Comet as Akai let out a weak chuckle.
"I can't, she keeps making weird noises-AAAAGCK!!!" Akai was cut off when she bit down harder.
Akai slowly breathed, in and out, as his body began to grow cold.
"...Listen, Nezuko." Akai said weakly. "I don't care if you drink my blood. Go ahead and take all of me." He sighed breathlessly. "But calm your heart. Get a grip on this power." He felt her loosen her grip on him ever so slightly. She was listening to him. "If you don't, it'll cause more damage to you than you could comprehend." He said. "As it did to me." He grew quiet for a moment, before speaking.
"I'm already dying." Akai confessed.
This is what he had told Aoi. And this is why he had taken such a drastic measure in ensuring his fellow slayers would grow stronger.
"My body's reached its limit, and I most likely won't live long." He slowly began to choke, as tears welled up in his eyes. "I was planning on pouring the rest of my lifespan into taking down at least a few of the Upper Moons, but this is fine." Akai sighed. "Just from this fight, I can tell that you're stronger than me, Nezuko. And I have faith that you can finish this..." His breathing grew weaker as Nezuko stopped drinking. The humanity returned to her eyes as she let him go, letting him settle onto the ground. She rose to her feet, keeping her eyes on him as she saw his chest still.
She lifted a finger to her mouth and bit into it, opening up a small wound. She held the finger over Akai, letting a drop of her blood fall into his mouth.
"Get up." Nezuko finally spoke.
"Oh, that's good!" Akai immediately came back to life and sat up, envigorated by a single drop of her blood.
"If you're gonna die, die protecting my brother. You idiot." She suddenly seemed a lot more mature. And... Older? Now, she looked like she was older than Tanjiro, right around Akai's age (physical).
"Hm." Akai healed the bite on his neck and rubbed the spot, a slight blush on his face.
"Pervert." Nezuko turned and glared at him.
"Hey, you were the one sucking me-" He was cut off as Nezuko roundhouse kicked his head, sending him into the dirt once again.
"Teach me more of how to use this power before you kick the bucket." Nezuko turned away, walking away from Akai's barely conscious body.
"Okay, cool..." Akai gave her a thumbs up, head still in the dirt.
"..." Tanjiro was utterly dumbfounded by what had just happened.
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Some trivia. This one contains some important info so you might wanna read it.
Trivia One: "You didn't win, bastard!!!": Despite how one-sided the battle between Akaza and Akai appeared in the Infinity Train incident, Akai was actually quite lucky. Had Akaza not been halted by the sun, he would have followed up after his Chaotic Blue Metal Afterglow and killed Akai.
Trivia Two: The Pink Comet: The awakened, feral form of Nezuko is a non-canon, altered version of the form she used against Daki in the Red Light District arc. This form she takes is actually that exact form but had its power mutated and multiplied even further by Akai's power; courtesy of his blood. If refined, this form might even surpass a full-power, peak condition Akai Suisei.
Trivia Three: Akai's Mortality: Akai Suisei's power placed an incredible tax on his body. His ability to heat his body, coupled with other powers such as the blood manipulation demon art, Red Ashura, and the Golden Ripple, has weakened his body despite his healing. His lifespan has been reduced, and if he is to use this power further, It may decrease further.
Okay, that's a wrap.
Wait, hold on...
....Oh, what's this?
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Kyojuro Rengoku sat on the floor of his home, facing another person.
"So, what do you require of the flame pillar?" He asked with a smile. "I was not expecting a girl from the Butterfly Estate to seek me out."
Aoi Kanzaki swallowed a lump in her throat, looking him the eye and speaking:
"Please train be in the Breath of Flame." She said.
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Okay, now it's done.