Takamura Rides the Mugen Train (Sakamoto Days/ Demon Slayer)

It was a funny thing, knowing Death was so close. It brought the mind to strange places, even in the heat of combat, with life on the line in a desperate gamble for time against a monster that spelled certain death. Rengoku had only a single thing on his mind. A question.

Clack. Clack. Clack. Clack.

Who was walking about right now? It was a question laced with confusion as he dodged fists that might as well have been blurs, as he swung furiously, deflecting bone-jarring blows and elbows that crushed boulders with the pressure released from near misses.

Clack. Clack. Clack. Clack.

Every second his death drew closer with the sounds of Akaza's mad laughter. He doubted Tanjiro could notice or observe it, but he was growing weaker. His dodges, mere inches. His cuts not as deep as they used to be. His slashes losing power as he bled all over.

"You're growing weaker." Akaza noted, sadness tinging his tone. "Accept my offer, Kyojuro! Let us do this for eternity!"

Rengoku's response was to jump back. His grasp over Total Concentration Breathing was loosening, his limbs were slowly getting heavier. Only his strict training and inexhaustible heart fire kept him standing. Rengoku was going to die, and he knew that, but he would not die for nothing. He only needed a few minutes more. The sun was definitely going to come up soon. All he needed to do was buy time for the demons most hated enemy.

Clack. Clack. Clack. Clack.

He shifted his stance, and for what felt like the first time, he watched Akaza's unnatural eyes widen in surprise, before he called out gleefully as he slipped into his own strange stance. "Come at me, and let us fight till our hearts burst, Kyojuro!"

Rengoku let out a single steam-fueled breath, his remaining eye widened until his pupil was a mere dot in his iris. This was him at his best. He had pulled out a hundred and ten percent of himself. All that was left was to strike.

"Ninth Form: Rengoku!" He screamed out, and in a blur of motion, and explosion of power that completely obliterated the ground around him, Rengoku disappeared, only to appear in front of Akaza faster than the Upper Moon demon could comprehend. He swung down, blade superheated by the speed of his movement and by his breathing style. It cut into Akaza's neck faster than he could react, but he only got halfway before the Upper Moon reacted. Akaza's hand caught the blade, redirecting the slash down his well-carved and tattooed chest, and Rengoku knew that he had missed his opportunity.

Death was finally coming.

Akaza's hand shot forward, his body instinctively going for a kill shot in response to his near death and Rengoku closed his eyes, knowing that in a split second he would be dead. Yet a split second passed, then a second, and finally another. It took Rengoku two and a half seconds to come to the conclusion that he still lived. Which left him with another question. Why?

He opened his single working eye as dust from the explosive movements they had taken slowly settled, and he looked to the ground and was greeted by Akaza's forearm, neatly cut below the elbow, lying on the ground, fist clenched in the blow that should have formed a hole in his midsection.

Rengoku felt Akaza's surprised eyes on him, and as one, the two fighters allowed their gaze to trail to the sword that still hung motionless, frozen after its movement that had relieved Akaza of his arm. The blade was nothing special, three feet of steel, lacking the multicolored hues of a Nichirin blade. The tsuba was just as bland, a round disk that separated the blade from the hilt, unlike the oft-exaggerated features of a Hashira's blade.

Finally, their gazes moved to the figure that held the blade. He was an old man. Short white hair carefully smoothed back. His figure was hunched over, but it hardly detracted from the fact that he was a small man. His features were serene, his white bushy brows at ease and his eyes shut like he was sleeping. He was dressed in a suit, one of those foreign wears that were slowly making their way into mainland Japan, and in his free hand was the sheath of his blade.

The moment Akaza laid eyes on him, he cracked the ground in his haste to retreat, jumping back twice, leaving two craters from where he had moved rapidly until he was an appropriate distance away. For the first time, Rengoku saw what seemed like fear or worry on the Upper Moon's face.

"W-Who are you!"

Akaza questioned, his attention and interest in Rengoku long forgotten at the entrance of the stranger. If Rengoku had been uncertain about the worry on Akaza's features before, the way his voice cracked made that a certainty. Akaza, The Upper Moon Three demon, was worried. If Rengoku were not currently worried about dying of blood loss, broken ribs, fractured shins, and a cracked tibia, forcing him to use inhuman will to keep himself standing, he would have laughed.

Instead, he watched the old man slowly slip the sword back into its sheath, uncaring of the Upper Moon that was questioning him, and once more began to walk forward to the steady clack of his feet hitting the hardened ground.

Rengoku used confused eyes to trail the path the old man had taken, his eyes following the footsteps imprinted into the ground until they reached the derailed train. Was he a passenger on the train? The old man had walked in a straight line, and it seemed that instead of saving Rengoku, he had simply decided to separate the duo blocking his way.

It was an insight that Akaza did not seem to grasp, because the moment his injuries healed, his newly regrown forearm shooting out from where there was nothing, he flexed it, and a second later, he blurred in front of the old man.

"I'm talking to you, you basta—"

Something dropped as Akaza was cut off. It took Rengoku another full second to realize the previous thought had been literal. Akaza's head lay on the ground, his body still as blood sputtered from his neck. Rengoku's wide eyes met Akaza's. He had not even seen the unsheathing of the sword, nor the swing. The only thing he could see was the result. For the second time, the old man slowly sheathed his sword as he continued his walk, pace unbroken.

Rengoku's heart soared in relief until he saw Akaza's body move to lift his head from the ground and place it once more on his neck. At once, the thin line that separated the two body parts sealed.

A Nichirin blade would have ended it.

"A retired Hashira?" Rengoku whispered to himself in surprise. Meanwhile, Akaza turned around, his features twisting into anger, hate, and rage, yet underneath that was a joy. A mad joy for battle.

"Turn back and fight me properly, you senile bastard!" the Upper Three demon roared, then he blurred forward, his movement so fast that the slowly weakening Rengoku could hardly catch it. For the first time, Rengoku saw the old man move, and only because he suddenly stilled, then tilted his head to the side the same moment Akaza appeared behind him, fist outstretched to crater his head.

The force of the missed blow sent the old man's short white hair flying. The old man spun in response, movement slow enough for Rengoku to catch. Then once more Akaza's hand was sent flying. Before Akaza could do more than grit his teeth, his knee buckled and a line opened across it. The strike had been too fast for even his monstrous reflexes to guard against. The limb gave way, folding beneath him, and before he could stabilize, his other arm went spinning into the dirt.

Rengoku watched with bleary eyes, wondering if he was truly watching an Upper Moon be butchered like a peice of meat or if it was a hallucination caused by blood loss. The roar of anger that followed was loud enough to tell him which was which. The old man stood still, in what Rengoku was guessing to be surprise.

"Damn you!" Akaza howled, his fury mingling with laughter, as blood hissed from his regenerating stumps. "Yes… yes! This is what I crave!" The demon lunged again, uncaring of the fact that he had lost both arms and one leg hung limp. His body twisted unnaturally, shoulders cracking as he shot forward to shoulder-check him, trying to find an angle the old man couldn't answer.

But the old man moved like water through gaps, with every step sounding like a gentle clack that echoed through the night. He took a step forward, slipping past Akaza's desperate lunge and shoulder check, then he made a turn of his wrist, followed by a small pivot, and Akaza's ribs split open, steaming blood sent flying.

Another flick of the old man's wrist sent his sword disappearing into a blur once more, followed by skin rupturing as the tendons in both of Akaza's leg unraveled, sending him sprawling face-first to the ground.

However, Akaza was an Upper Moon demon, the Upper Moon Three, and in his veins ran Muzan's undiluted blood. The demon shot forward again, a scream of joy on his face. His arm had already healed, his tendons were halfway healed by the time he propelled himself off the ground with the aid of his hands, and by the time he clashed with the old man once more, he was whole again.

The dance continued for seconds, the tale a blur to Rengoku's slowly dimming senses. What was clear was the aftermath. The ground was littered with pieces of Akaza, his own flesh scattered and splattered around like discarded offerings to the slowly rising sun.

Yet the Upper Moon only laughed louder, blood dripping from his mouth, uncaring of just how close he was to death. They slowed down before coming to a stop, revealing the mangled figure of Akaza, but already the Upper Moon was healing once more, while Rengoku could see the old man take a single breath, the only sign of his exhaustion.

"Finally! Finally, an opponent who cuts me down! I will give you the same offer, old man. Become a demon! Fight me forever!"

The demon shot forward, and one strike from Akaza landed, a glancing blow that tore through the ground where the old man had stood a half-second before. The shockwave rippled through the battlefield, bending trees and tossing rubble. Even then, the old man's blade flicked once more, opening Akaza's cheek and knocking out several of his teeth.

Blood poured, then boiled away as the flesh re-knit.

The demon spat, grinning through the gore. "Why won't you answer me? Say it! Say you live for this too!"

The old man did not deign to reply. His eyes never bothered to open. Instead, he simply sheathed and unsheathed, his blade whispering death as it left its scabbard. Then he disappeared into a blur and Akaza followed a split second later. They clashed once more, and blood spilled again, the ground they fought on long stained crimson.

Rengoku's chest heaved as he watched, half-delirious. It was a dreamlike sight, an endless storm of killing intent answered only with silence and the swing of a blade.

The battle raged until the eastern horizon glowed bright with light. Rengoku, straining through blurred vision, noticed it before Akaza did, the creeping warmth, the gentle burn at the demon's skin.

When the first true ray struck, Akaza snarled, panic flashing across his face. His laughter broke into a frustrated roar. "Damn it! DAMN YOU!" He skipped back instinctively, far enough to jump into the shade. The old man simply paused, surprised by the sudden retreat.

Rengoku watched as Akaza looked at the old man, he could feel the thought going through the demons head as he thirsted for that thrill and joy of the combat he had experienced over the past few seconds. Then Akaza unconsciously made to step forward once more, out of the shade, but his toes caught fire. Rengoku's eyes widened further in shock as Akaza took yet another step, eyes wide in battle-forged ecstasy and trance, uncaring of the death that called to him. He took another step forward, and his shin ignited, caught aflame alongside his pants.

Rengoku looked on, eyes wide in surprise as the demon almost took another step, but suddenly froze mid-air. Akaza's eyes rolled back, turning white for a split second, then he was back, and he took a step backward, away from the old man, away from the battle, and away from the upcoming dawn.

There was a sadness to his features, one that transcended sorrow and went into a deep melancholy. Akaza looked at the old man and said, "My master will not permit me to do battle under the sun." He was almost... Apologetic. "Next time, old man, next time we fight until nothing remains! Even if it means self Immolation."

Only then did he look at Rengoku, and with a final smile, he tore himself free of the battlefield, scattering dirt and broken stone as he vanished into the trees, deeper into the forest, fleeing the coming dawn.

The clacking steps slowed. The old man had moved to follow but had stopped walking after a thought. He turned his head slightly, as if acknowledging the mangled demon pieces strewn across the floor that were spontaneously combusting under the sunlight, and then to where Rengoku knelt, his body broken and held up by his sword, but his spirit still burning.

Rengoku's vision dimmed.

He could hear Tanjiro running, shouting something that did not register as words, and there, in the haze, the last thing he saw before his eyelid dropped shut was the old man turning away, blade slowly slipping into its sheath once more as the stooped figure calmly walked into the distance.

A/N: A rough draft and probably a one shot. Wrote it on a stretch and wanted to share out of pure excitement.

This was supposed to be a JJK crossover, with Takamura appearing when Sukuna prohibits everyone from taking a single step. That's where the sound of Takamura's footsteps would've been chilling. He would probably have been mistaken for having a Heavenly restriction, which would take the story to interesting places. Unfortunately, i've one too many JJK fanfics already. So you get DS instead.