The Multiverse Voyager

(A/N- I made a mistake last chapter. It's actually Kazuhiro Kyojuro, not Kazuhiro Rengoku. I'll reveal his BDA later on)

(A/N- The drunkard father of Rengoku is called Sengoku.)

Also, as an apology for a late upload, I've uploaded two chapters back to back

[Chapter Start]

This guy is definitely an ancestor of Rengoku. His eagerness, sense of justice, and exaggerated way of speaking: even the desire to protect civilians and the responsibility of the strong to protect the weak. everything matches Rengoku's mannerisms.

'I feel bad for Sengoku if they ever meet'

Kazuhiro strode toward the cavern's entrance, each step radiating enough heat to make the stone beneath his feet crack and smolder. I scrambled to keep up, the oppressive heat forcing me to shield my face as the air shimmered around him like a mirage. The glowing flame markings on his face pulsed with each step, casting eerie shadows along the cavern walls.

Kazuhiro Kyojuro placed his hands on the wall, opening the doors leading out. He then stepped out the gates-

*thwip*

-and fell on the ground immediately.

"Huh?"

"I seem to have fallen"

I blanked out as I saw Kazuhiro loudly say while smiling as he flopped face down as soon as he stepped out of the cave's gates. I rushed to him, helping him back up and leading him back to the cave, where he slowly regained his vigor.

"Hmm," he muttered, stroking his chin. "It seems I haven't eaten anything for a few centuries. My Blood Demon Art has been passively converting the heat in this cave into sustenance. Outside of this environment, my body will perish."

"Huh"

How anti-climatic considering that epic speech he gave. And here I was thinking I made the final battle against Muzan easier by adding someone as strong as Upper Moon 1.

"If it's about heat" I continue "I could probably make a portable thermal generator or something, I guess something like a portable heated jacket, but hotter? Hmm, I'd need to find or create fabric strong enough to handle that amount of heat. Not to mention, I'd need a power source that can be easily replenished in this era, maybe something solar-related. Not that would kill you."

"I do not understand your words," Kazuhiro interrupted, tilting his head in confusion.

"Don't worry," I sighed. "I don't think anyone here would."

I sigh as I scratch my head. I had hoped to use Kazuhiro's help to kill the demon controlling this forest, but that plan isn't going to work anymore.

"You're thinking about that demon, aren't you?" Kazuhiro asked, his gaze steady.

"Yeah," I admitted. "I'm still trying to figure out how to kill it."

"I see," Kazuhiro said as he nodded his head. "You should give up on that for now"

"Give up?" I said incredulously "Aren't you supposed to motivate me to go past my limits? Something like set my heart ablaze?"

"Ablaze? With Water?" Kazuhiro scoffed "From the way you speak, it seems you know my descendants in the current era. I'm surprised my bloodline is still recognizable, I figured it would have a different surname by now"

"It's surprising, yeah," I said, waving it off. "But that's not important. Do you seriously expect me to give up?"

"Don't say it like that" Kazuhiro said, shrugging his shoulders "Think of it as a… strategical retreat. Even if you somehow manage to defeat an entire forest and reach the demon, the demon is simply too powerful for you to defeat at your current power"

Kazuhiro looked in a certain direction upwards "That demon has consumed all life on this mountain, including the demons present here. There were around 45 demons present here, but they're all digested now."

"Furthermore," Kazuhiro said furrowing his brows "Not only did that demon gain immense power from eating that many demons, but it has somehow absorbed the Blood Demon Arts of the few demons that had one."

"It absorbed Blood Demon Arts?" I said in surprise "I didn't know demons could do that. Doesn't that mean that any demon can just eat other demons and gain multiple Blood Demon Arts?"

"No," Kazuhiro said shaking his head "Normally demons can't kill or eat other demons, only sunlight or a Nichrin Blade can do so. I suspect it's this demon's Blood Demon Art that has allowed it to do so."

"A demon with multiple Blood Demon Arts…" I muttered, piecing it together. "Shouldn't the Slayer Corps have already sent someone to handle a threat like this?"

"I'm not aware of what the Slayer Corps are doing right now," Kazuhiro said shrugging "But they have made the right move. This demon has merged with the entire forest rendering it incapable of movement. Furthermore, this mountain is surrounded by Wisteria, something that the demon cannot merge with. It's essentially trapped in this forest."

"There are already a lot of demons to deal with," Kazuhiro said stroking his chin "Rather than risk manpower on an immobile demon, it would be better to just seal off the area and hunt other mobile threats instead."

"I see" I muttered "That makes a lot of sense"

'If the demon has multiple Blood Demon Arts and has merged with an entire forest then it's most definitely not a demon I can take care of right now. Even Enmu, the strongest lower moon, was only able to merge himself with a train, and that after he received a huge dose of Muzan's blood.'

'Furthermore, just like Enmu there's no guarantee of his neck still being on his main body, for all I know it could be scattered anywhere in this entire forest. I don't know if this thing is as strong as an Upper Moon'

'But…it's definitely stronger than any lower moon'

"You're right," I said, shaking my head. "There's no way I can deal with that demon right now. I'm not strong enough to take on a Lower Moon, let alone something even stronger."

"I'm glad you're thinking logically," Kazuhiro said, nodding in approval. "Too many slayers let their pride as swordsmen cloud their judgment."

'Well, I was about to do just that,' I thought nervously, grateful I hadn't voiced my earlier plans.

"Still," Kazuhiro added, his tone sharpening, "with your unique body, defeating that demon might take you much longer than you realize."

I frowned, confused. "My unique body?" Then it hit me—he had used See-Through World to analyze me.

"Your muscles, ligaments, tendons, and skeletal structure are the greatest quality I've seen. Your potential is the highest I've seen till now" Kazuhiro said looking at me "But that is also slowing down your growth."

"Slowing me down?" I asked, trying to process his words.

Kazuhiro folded his arms. "Techniques like Total Concentration Constant don't give an instant boost. They work by gradually breaking down the body and rebuilding it stronger. But your body is so resilient that the process is far slower for you than it would be for an average slayer."

I opened my mouth to argue but stopped, realizing he was probably right. If Total Concentration Constant gave an instant boost then Tanjiro, Zenitsu, and Inosuke should have been as strong as the Hashira upon mastering it, but they were still far behind them in terms of power.

"And that's not your only issue," Kazuhiro continued. "Your strength will eventually outgrow your sword."

I glanced at the Nichirin Blade strapped to my side. "Outgrow my sword? What do you mean?"

"Over time, your strikes will become so powerful that even the finest Nichirin Blade won't be able to handle them," he said plainly. "It will either crack, shatter, or dull too quickly to be effective. Many slayers who reached your potential faced the same problem, but most solved it the wrong way."

"What's the wrong way?" I asked warily.

"They held back," Kazuhiro said, his voice laced with disdain. "They restrained their power, fearing they would break their weapons. That hesitation cost many of them their lives."

I clenched my hands as I looked down. Making a weapon capable of handling my strength wasn't that hard if I just went back to my world and made a support item, but what about later on in my journey?

I'll go to more worlds, I'll become even stronger when I absorb their physiques and learn their techniques. At that point, finding or making a weapon powerful enough to contain my strength would be nigh impossible.

"You have two choices," Kazuhiro said "You could get a weapon durable enough to handle your strength, the Stone Hashira of my time used a combination of a mace and an axe for his breathing technique once he faced the same problem."

"And the second?"

"Hone your martial arts, make your body the ultimate weapon," Kazuhiro said scratching his chin "This method might be better for you, but it's much riskier than the first option. Most slayers don't live long enough to hone their body to perfection"

"A martial art?"

Kazuhiro sighed, a look of disappointment flashing across his face. He folded his arms. "You mean to tell me that modern slayers rely solely on breathing techniques and their swords?"

"Well," I said, shrugging, "that's what I've seen so far. The breathing techniques are pretty versatile. I mean, they're powerful enough to stand against Demons, right?"

"I suppose it's to be expected," Kazuhiro muttered, though the disappointment lingered in his voice. "In my time, when breathing techniques were first introduced, demon slayers honed their bodies with martial arts. The Slayer Corps wasn't made up of just swordsmen back then. We had archers, dagger-wielders, martial artists—people who fought with bows, gauntlets, axes, anything you could think of."

"Then Yoriichi appeared," Kazuhiro continued, his voice softening with respect. "He taught us breathing techniques, and because he was a swordsman, it was easiest to pair his teachings with a blade. But even then, some warriors relied on their bodies, using breathing techniques solely to enhance their physical prowess."

"Of course," he added, his tone growing grim, "those warriors eventually vanished. Back then, demons were everywhere. Slayers didn't have the luxury of time to develop entire martial arts or integrate them fully with breathing techniques. We fought to survive, plain and simple."

"So" I questioned, "Back then Martial Arts were basically like breathing techniques?"

"Somewhat like that yes" Kazuhiro nodded "They didn't grant a boost as great as breathing techniques though. Even a novice user of a breathing technique could fight off a master martial artist."

"Even so" Kazuhiro narrowed his glowing eyes at me, his expression a mix of frustration and pity. "Breathing techniques amplify your strength, speed, and stamina, yes. But they are not a substitute for discipline and control. Without a solid foundation in martial arts, you're leaving gaping holes in your combat style. A demon with experience will exploit those."

"I think," I said "it would be better for me to use my body as a weapon instead"

"Are you sure?" Kazuhiro asked. You have great talent in wielding weapons. You could just choose the safer route and still be powerful enough to fight off the strongest of demons with your potential."

"I understand that, but there are reasons for choosing this that I cannot tell you yet," I said as I clenched my fists "Besides, I could shatter a boulder before I could slice it with a blade"

"I see" Kazuhiro nodded "That might be good. Martial arts also break down your body and rebuild them. If you combine them with the Total Concentration Constant, you might grow at a very high rate"

"There are multiple martial arts you could take, but I am unsure if they are still around." Kazuhiro continued. "There was a Dojo that taught the Soryuu style. It could enhance the body into the ultimate weapon. Unfortunately, their main motto was to transform the body into a weapon as deadly as a blade and so, they refused to teach the Slayer Corps once they started wielding swords."

"That style is extinct, I believe some rival dojo poisoned them long ago out of jealousy," I said as I remembered Akaza's backstory from the manga. "The only remaining wielder of that style is Upper Moon 3 now"

"That's a shame" Kazuhiro sighed "If the dojo was still around, they would have loved to have you as their disciple. I'm sorry then, but I cannot help you in this case, I have no idea what is going in the world outside now"

"Don't you have martial arts dojos where you come from?"

"My place?"

"You're from outside Japan, aren't you?" Kazuhiro asked, tilting his head.

"I... guess you could say that,," I replied with a sigh. "Martial arts dojos where I come from are pretty much extinct. The ones that still exist are just flashy knockoffs meant for showing off."

Once Quirks emerged, martial arts basically went extinct. Nowadays, Heroes just stick to developing their quirks and only learn martial arts to help support their quirks. For someone like me who is quirkless, they may as well be useless.

"That's unfortunate, though I suppose it makes sense," Kazuhiro said thoughtfully. "From what I've heard, demons only exist in Japan, so the outside world never had the urgency to develop martial arts capable of supernatural feats."

He paused, then asked, "But how do they fight wars with enemy nations without proper training?"

"They've advanced their weapons," I said, recalling what I knew of history. "They have guns, cannons, and explosives—things that could probably kill the Demon King with ease."

"Really?" Kazuhiro looked genuinely surprised. "Then why hasn't the Slayer Corps acquired these weapons?"

"I don't know," I admitted. "Weapons that powerful are heavily regulated. Not to mention, the cost of producing them, like large-scale artillery or explosives, would be insane. Besides, trained Slayers with Breathing techniques are still more powerful than these for now"

'Though, that might change once missiles are invented in 1930, and the first nuclear bomb is developed'

"Right then" Kazuhiro then motioned behind him, changing the subject. "The metal trees are located at the border of the forests. Just leave from the exit behind me to exit the forest. You can worry about martial arts and weapons later, for now, you should go back to your teacher and set off for the final selections."

"Right" I nodded "I'll see you after I complete the final selections then."

Following Kazuhiro's directions, I made my way toward the path he had pointed out. Eventually, I reached a steep wall, its rugged surface rising sharply in front of me. Determined, I began climbing, my fingers gripping the uneven rocks for support. As I neared the top, I clenched my fist and delivered a powerful punch, breaking through the ground above. Sunlight poured in as I emerged from the underground cave, the fresh air brushing against my face.

As I stepped past the forest's edge, the heavy silence lifted, replaced by the gentle rustling of leaves in the breeze. The faint scent of wisteria flowers lingered in the air, a stark reminder of how suffocating the cursed forest had been. I turned around and glanced back at the dense canopy, shadows twisting and turning like they wanted to pull me back in.

"I'll be back," I muttered under my breath, clenching my fists. "When I'm stronger."

The path ahead sloped downhill, leading back toward civilization. With each step, I felt the tension ease, like the forest's grip was finally letting go of me. But my mind was still tangled up in everything I'd just left behind—the demon hiding in the mountain, the challenge of learning martial arts, and Kazuhiro, whose presence was as confusing as it was inspiring.

'I've got a long way to go,' I thought, the weight of it sitting heavy in my chest.

The sun sank lower as I made my way along the path. By the time night fell, I'd found a small clearing to set up camp. The air was cold, the stars unusually bright against the dark sky. I lit a small fire, warming my hands as the events of the day replayed in my mind.

'The final selection is waiting for me,' I thought, staring into the flames. 'I have to survive that first.'

The days blurred together as I made my way back to Mount Sagiri. The terrain was rough, but the familiar sight of the mountain in the distance pushed me forward. Exhaustion clawed at me, but I pressed on, determined not to waste any more time.

Finally, on the evening of the third day, I reached the foot of Mount Sagiri. The familiar silhouette of Urokodaki's house stood against the backdrop of the setting sun, smoke curling gently from the chimney.

Climbing the final steps to the porch, I let out a breath I hadn't realized I'd been holding. My body felt like it might collapse right there, but before I could knock, the door slid open. Urokodaki stood there, his expression calm but expectant, as though he'd been waiting.

"You're back," he said simply.

"Yeah," I replied, bowing slightly. "And I'm ready to prepare for the final selection."

He studied me for a moment, then nodded. "Good. Rest first. You'll need it."

I stepped inside, the warmth of the house wrapping around me like a blanket. My legs nearly gave out, but for the first time in days, I felt something close to peace. The real challenges were still ahead, but at least for now, I could rest.