On a fateful day, Rengoku had departed with Tanjiro, Inouske and Zenitsu to the Mugen Train Station in order to investigate the mysterious disappearance of passengers.
Unexpectedly, they had fallen into the trap of Enmu, the Lower Moon One of the Twelve Kizuki. Although, they almost died, they were able to successfully slay the demon, finally completing their mission.
Unfortunately, relief wasn't their portion today.
Right after the battle, a terrifying demon appeared.
Upper Moon Three—Akaza.
Rengoku had confronted him, almost matching the demon with magnificence and vigor. However, their difference in strength was soon revealed.
Rengoku was pushed back, a scar across his left eye.
Akaza smiled, clenching his fists.
"Exciting. You're the most exciting Demon Slayer I've ever fought. Although weak, you possess the vigor of a seasoned fighter."
Stretching forth a hand, he grinned.
"Why don't you become a demon? You humans are weak and fragile. Aging quickly and dying pitifully. As a demon, you can perfect your battle arts and grow stronger! Leave this useless human husk behind!"
He spoke as if preaching about utmost salvation.
Rengoku smiled faintly, holding his sword in a defensive stance.
"I won't trade my honor as a human for your so called longevity. So..."
Flames erupted from the sword, burning brightly.
Boom!
They clashed. Fists struck blade. Blade sliced flesh several times but the wounds instantly regenerated.
"Your honor is disgusting." Akaza frowned. "Being given a chance to become a demon yet you refuse!"
Rengoku groaned. "Flame Breathing - Ninth Form."
He glowed, flames cloaking him like a dying neutron star.
"Rengoku!"
Akaza also struck, his fist as fast as lightning.
Bang!
Two powerful people. One demon, one human. Both severely injured.
Tanjiro could only watch powerlessly as Akaza's arm tore into Rengoku's torso while the latter's sword was dug halfway into Akaza's neck.
"You—!"
Akaza growled, stunned.
He didn't expect the Flame Hashira to be this strong! Almost killing him, and even struggling to finish him off on death's door.
However, he wouldn't be an Upper Moon if he were to die to an Hashira so easily. Still, the sun was close to rising. He wouldn't be able to consume this Hashira.
"What a shame."
He clicked his tongue, then dislodged his arm from Rengoku's innards—blood spurting in its wake.
With a powerful snap, he broke the sword embedded in his neck and fled for cover.
"Akaza, you coward!!!"
Tanjiro screamed in raw rage as he watched the demon leave while Rengoku lay at death's door.
Rushing to the Hashira's side, he said panicking.
"Try to control your breathing. It should stop the wounds from bleeding. The Insect Hashira will find a way to heal you."
Inouske also knelt beside the Hashira, growling in sadness.
Looking at the three; with Zenitsu unconscious, Rengoku smiled.
"Don't be sad. Everyone has their own time to go. Tanjiro, grow strong and protect your sister. You demon slayers are still young so..."
"... set your hearts ablaze."
...
The news about the death of the Flame Hashira rattled all the Demon Slayers. There was no demon slayer who could boldly claim that they never heard of Rengoku.
And his death, was a painful reminder of how deadly the issue of demons were.
During the meeting held after his death, Kagaya stressed out his grief and the need for more training even among the Hashiras.
...
"Lord Akaza!"
Douma called out in a happy voice as he spotted the demon standing on the roof of a warehouse in solitude.
Turning to him with a cold look in his eyes, Akaza scowled.
"How did you find me?"
Douma laughed softly like a bell.
"I heard the news that you killed an Hashira! So marvelous! Lord Akaza's strength is simply stunning!" He praised in what he believed was genuine admiration.
Akaza only turned away—because he knew. He had struggled to kill an Hashira, almost dying, even. Yet, Douma had played under the combined assault of three Hashiras.
Was the disparity in their strength that massive? Or was it the Flame Hashira who was just that strong?
He didn't know. However, he would never accept such a praise from Douma. Someone who was the polar opposite of his principles. Douma believed that eating weak people who were struggling to survive, especially women, was an act of saving them.
However, Akaza knew how twisted such logic was. Killing people to save them? Nonsense!
Also, Akaza would never eat women, a taboo deeply imprinted in his mind. Although he didn't know why, he would always adhere to it.
He wanted to curse Douma, but suddenly remembered Lord Kokushibo's words and swallowed his pride.
"You should leave. I'm not in the mood for you."
He said dismissively.
Unexpectedly, Douma turned back, smiling faintly.
"Alright. See you later, Lord Akaza."
Akaza was stunned, surprised Douma actually left instead of being persistent. Was something wrong with him?
.....
During the next few days, Douma visited his followers frequently, checking up on their work and encouraging them with smiles and rich, succulent dinner.
However, there was a problem.
He was already longing for Shinobu again.
'Something is wrong.'
He thought with a rare frown.
For the first time, he questioned his own feelings—if he had any. Was it attachment he felt towards the Hashira? Or was it curiosity? Interest? Because she was the one who made him feel his first emotion—loss.
"Ah... So troublesome. I don't want to think." He sighed in mock dismay, resting his hand on his face.
His rainbow colored eyes looked up to the bright moon, his usual smile absent.
'Why did I come to the past? Does it truly have nothing to do with Muzan?'
He thought solemnly.
Still, he could only say...
'Since I'm here... There's no problem changing a few things.'
That night, he descended on a nearby village and gobbled up all the homeless people painlessly.
'Poor souls.'
He sighed in what he believed was pity.