The Multiverse Voyager

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Total Concentration Constant

An advanced version of Total Concentration Breathing, this is a technique mastered by only the Hashira and their Tsugoku. Unlike standard Total Concentration Breathing, which is only used during combat, this version demands the user to continuously maintain the heightened state of breathing—during battles, daily activities, and even while resting or sleeping.

The technique is a grueling test of both physical endurance and mental discipline. It amplifies oxygen flow throughout the body, enhancing physical capabilities, accelerating healing, and pushing a Demon Slayer's body beyond its natural limits. By constantly engaging in this breathing technique, the user can adapt to endure prolonged combat and refine their control over Breathing Styles.

And I've mastered it.

A technique that even Kinoe-ranked slayers are unable to master, has been mastered by a slayer in training. Most of this credit however goes to the gas mask and the combination of physiques of my world and the demon slayer world.

The people in MHA have ridiculous physiques, they can easily match lower-ranked demon slayers without their quirks if they just hit the gym. People like Knuckleduster could even take on former lower moons or maybe even the weaker three lower moons despite not having a Quirk.

The people in the Demon Slayer world don't have bodies quite that ridiculous, but they do have minor healing factors. While the MHA physiques were focused on power, this was focused on agility and survivability. And while not that powerful, the speed and power of this physique were still incorporated in me.

When you combine these two, overwhelming power, speed, and minor regeneration, your body adapts to the harsh conditions of continuously maintaining breathing techniques. If I attempted this with only the physique of this world, my lungs would have burst from this training method. 

That's not to say this training method is dangerous, but rather my lungs would have first needed to adapt to the strain of Total Concentration for a few months. The three squad of main characters were extremely talented, so they only required a month to do so but there's no guarantee my body would have adapted that quickly.

What I can't explain is how I mastered breathing techniques and basic sword techniques easily. Ever since I transmigrated into this body, my mental processes seemed to have elevated by a lot, and then elevated even further when I came into this world.

I don't have a concrete answer for this, but I have read a lot of Chinese fanfictions so it's most probably due to me transmigrating in this body and the souls merging. No, the souls didn't merge because I still possess no memories of this life.

Is it because the people in MHA are generally much smarter than those in my normal world? Geniuses did exist in my last world, but the students in the support course, even the average ones, could somehow make ridiculous inventions and hero suits while only being teenagers. 

I guess it just boils down to the physiques again, what with my intelligence and mental comprehension increasing when I entered this world. Not to mention, the previous AYD that inhabited this body wasn't exactly average; he could easily keep up with the top support course students.

That does bring some questions to my mind. What if I entered a world with ridiculous physiques? Like the One Piece world? I don't even need to stay in that world for a long time, my physique instantly upgraded the moment I stepped into this world. I could easily defeat Muzan Kibutsuji if I just trained for a year or two without even using breathing techniques or Haki.

And most importantly, what happened to the AYD who previously inhabited this body? Where did his soul go?

…

I'm too weak and inexperienced in matters of the soul to answer that question and while I could just pop in and out of One Piece and gain overwhelming power, my fight with Sabito taught me something really important.

Speed and power are nothing without experience. Despite having mastered Total Concentration Constant, I could neither slice a boulder in half nor defeat Sabito. His movements were extremely fluid and his instincts were top-notch, relative to mine at least. If I tried to slice the boulder in half, I would have instead broken it into multiple pieces and shattered my wooden blade.

If I remained inexperienced and relied on absolute power instead, I would eventually hit a ceiling. This is one thing those badly written stories never explored like The Hidden Class Gravity User Webtoon. It sounds similar to the Gravity Mage level-up system on Webnovel but is completely different. 

The MC of Gravity User, Ikoma Ichinose is a typical revenge-type regression MC. Weak class, got kicked out of his party, fought an extremely strong monster and died, but was reborn back in time, and used advanced knowledge of the future to get an extremely OP class.

Except this guy fucking butchers his class. His class is, as you can guess from the title, a class that can wield Gravity itself and you know what he does with it? He boosts his physical attributes. He makes himself lighter by reducing gravity and then at the last moment, he increases gravity to increase his attack power.

That's it. That's all he does the entire series. No increasing gravity in an area to slow down opponents, no flying by removing gravity, no black holes or whatever, just increasing and decreasing his weight. Fucking Yuki Tsukumo could do more and she can't even manipulate gravity. Such an amazing ability, yet such terrible execution.

If I just gained abilities left and right with no experience in using them, I'd be stuck with only the most basic usage of them. It's just like All For One who should really be the strongest in the verse with his ability to steal other quirks, but he doesn't train them at all and only sticks to the most basic applications of the quirks. A basic stockpile Quirk like One For All, even with 7 multiple quirks should realistically have no chance against All For One if he even trained slightly with his abilities.

I mean think of it logically, OFA is, at its base, a stockpile quirk. AFO could have just stolen a random stockpile quirk, or hell, multiple stockpile quirks, and gotten an even stronger version of OFA but his extremely unhealthy obsession with his brother limited him from having such thoughts.

I don't want to be like that. I need to experience hardships and, as much as I dislike it, at least a few near-death experiences. This is why, despite almost dying to the forest demon, and the demon possibly being as strong as a lower moon, I am still here, standing in front of the metal tree, preparing to re-enter the forest.

Now that I possess advanced sword skills and Total Concentration Constant, I should be able to fare better than last time. The demons in this forest, or the multiple demons in this forest are extremely strong, but that is the exact reason I am re-entering this forest.

The air in the forest felt different this time—a heavy, oppressive stillness that clung to every breath. The moon hung low, casting faint silver streaks through the dense canopy. I stood at the edge of the treeline.

I took my first step into the forest, the faint crunch of leaves beneath my boots echoing ominously. Almost immediately, the scent hit me—that sweet, sickly aroma that had caught me off guard the first time.

"Not this time," I muttered, pulling a cloth mask from my pockets and tying it over my face to filter the air. My lungs were stronger now, accustomed to the relentless strain of Total Concentration Breathing.

The first few steps into the forest were uneventful, but I could feel it—an unnatural stillness, like the forest was holding its breath. The faint rustle of leaves caught my attention, though the air was dead calm.

I leaped from tree to tree like a cat, making sure not to stay on a branch for long. Unlike last time when I was burdened by my heavy training armor, I easily swayed from one spot to another with ease. 

Then it began.

The trees shifted, their branches creaking unnaturally as they swayed without wind. Roots writhed under the ground, their movements causing the earth beneath me to tremble. I immediately jumped off the branch I was on as it came to life.

"So, we're starting with this again," I muttered, unsheathing my katana.

A root burst out of the ground to grab my ankle, but I was ready this time. Easily dodging the root I looked around me as I noticed a huge amount of roots and branches gathering around me, easily more than the last time.

"Water Breathing: Second Form—Water Wheel!"

I leaped into the air, my blade carving through the root in a graceful arc. The severed piece writhed before disintegrating into ash, but more roots burst from the ground, their jagged edges aiming to ensnare me mid-air.

"Water Breathing: Ninth Form—Splashing Water Flow, Turbulent!"

Twisting my body, I spun through the maze of roots, weaving between them with fluid precision. My movements were sharper, faster, and more controlled than before. I landed lightly on the forest floor, but the trees were relentless. Branches swung down like bludgeons, forcing me to roll and parry with my blade.

I readied my blade but immediately jumped from my initial spot as multiple hands made out of roots burst from the ground at my initial position. Did the demon somehow sense me getting stronger and increase his firepower?

Good for me.

I grab the ground, and with strength that is impossible for the humans of this world, I tear out a large chunk of the ground and slam it onto the hand, splattering it into tiny splinters. I dodge the splinters as I cut the roots around me.

More hands come out of the ground, but I cannot be bothered to deal with them. These things won't stop coming until I kill the demon controlling the forest. The most probable location for the demon is somewhere deep in the forest.

Roots and branches surround me as multiple hands slam down on me. I ignore the approaching threats as I crouch down, placing one leg behind the other as I squat down and place my hands on the ground.

"Water Breathing: Twelfth Form-Ascendant Torrent"

With speed surprising even me, I let out a huge boom, the force of the form splattering all the wood around me as I leap with immense speed. The world around me blurs as I dash through multiple trees.

I eventually stop for a brief moment as I catch my breath and look around me. One of the disadvantages of this form was that I needed to stop for a bit to recharge

The density of the trees surrounding me increased a lot. A moment after I analyze my surroundings, the trees come to life again.

I ready myself again, but notice the approaching roots and branches seem to slow down. That's weird, I'm sure that I'm deeper into the demon's territory, so why is his control over the forest getting weaker?

A loud rumble occurred as the earth beneath me split. Somehow the speed of the earth splitting seemed to increase with me unable to react to it, as opposed to the branches and roots slowing down. I immediately jump upwards using the walls of the ravine as I kick them with force.

The earth's rumble settled into a constant vibration, and I felt it—subtle shifts in the forest's energy. The branches and roots slowed their frantic assault above, but the ground continued to quake beneath me, as though the demon was shifting its strategy.

I stood still for a moment, catching my breath and analyzing the changes. My body screamed for rest after the sheer force of the Twelfth Form, but I couldn't afford to stop. The branches overhead moved sluggishly, and their strikes were slower and less coordinated.

"Interesting," I muttered. "Is there something stopping the demon blood art regarding the trees but not the ground? Maybe, I overshot the demon controlling the trees and accidentally arrived near the demon controlling the ground instead."

The ground beneath me trembled violently, roots coiling and uncoiling like snakes. They were faster now, surging toward me with renewed vigor. I leaped to the side, narrowly avoiding a jagged root that erupted from where I'd been standing.

I couldn't rely on sheer power right now. Not until I recovered enough to unleash another high-level form.

"This confirms that there are multiple demons," I say as I grin, crossing my arms with blade in hand. "Then, I should be able to kill the demon controlling the ground right now"

I dashed forward, weaving through the roots as they lashed out in erratic patterns. My focus narrowed. Each step was deliberate, my breathing steady despite the chaos. Total Concentration Constant allowed me to maximize my stamina while my body worked overtime to recover.

"Let's see," I said while avoiding the splitting ground and multiple roots. "Logically the demon should be further ahead, but I overshot the demon controlling the trees which wouldn't have been possible if he had been on the path"

"Then that means," I say sheating my katana as I ready my fists "the demon's either underground or above in the air, but it can't fly with its blood demon art"

A hand comes out with surprising speed out of the ground. Then other and another and eventually I get surrounded by the wooden hands. However, instead of being scared, this only relieved me.

"Looks like I was correct," I say as I raise my fist "The demon is underground"

And with humongous force, I slam my fist down on the ground.

"SMASH"

The ground beneath me pulverizes making a small crater. The shockwaves break the wooden hands apart as I grab the ground, uprooting the hands along with it and throw it away in a random direction uprooting multiple trees.

I feel a warm liquid sensation as I breathe and stare at my shaking hands. Looks like even with the enhanced physique I can't go on recklessly using such power.

The forest suddenly howls, releasing a sound so loud it forces me to close my ears. The uprooted trees grow wooden legs as they march towards me, and the ground around me in a large area completely disappears. Multiple jagged branches strike at me, forcing me to unsheathe my Katana as I cut them apart.

Why did the intensity of attacks increase this much? And that's not it.

"The root,s" I say watching the sliced roots multiply "They're multiplying the more I cut them"

I frown as I raise my sword. "That removes using large-scale attacks, and now I have to somehow avoid the roots too?"

"I was too reckless" I curse as I strike apart the jagged branches. The slower pace made them seem less threatening at first glance, but something about their rhythm felt unnatural.

"Wait," I say as I look at my hands " Is the smell of blood exciting the demons?"

I mentally curse at myself. Of course, these were demons that hadn't eaten in a long time. Showing them blood was akin to showing a starving beast a piece of meat.

I immediately slice apart a wooden hand as I stare at the multiple hostile objects approaching. If I continue like this, I'll drain myself completely without doing any significant damage to the demon.

I looked everywhere trying to find something to help while dodging and slicing apart all the roots, but every time I cut one, two more seemed to grow in place instead.

I leaped back to avoid a rising root, spinning mid-air to land on a narrow patch of solid ground but bumped my back against the branches. My eyes instinctively darted upward, drawn to the sky. The gaps in the treetops were smaller than before.

"Wha-?" I stop mid-question. The realization crept over me like a cold chill. This wasn't an attack. The branches weren't meant to lash out or strike me like the roots below. They were organizing, forming a barrier—a woven cage of wood and shadow that would make escape impossible.

"They tricked me," I said as I heaved, looking around me. Everywhere I saw, there were interwoven branches and roots blocking off the exit. The demon wasn't merely toying with me. This was strategy. A trap designed to corner me, to eliminate all routes of retreat. While the hands below distracted me the branches above worked in making a prison.

"Fuck, he purposefully made it seem like his control over his blood demon art had weakened so he would make me drop my guard when the entire time it was just making a cage to trap me. There might not even be multiple demons for all I know."

My breaths grew heavier, though not from exertion. I tightened my hold on my katana, feeling the weight of the situation press down on me like the canopy itself. If I let this continue, the forest would swallow me whole.

I bring my hands down to my bag trying to bring out a UV flashbang, but frown as I feel it missing. I look around, not noticing the bag anywhere. 

"My only hope, gone"

I backed up, the roots and branches slowly slithering towards me as if mocking me. I looked around everywhere, but there was no escape route anywhere. Since this demon could control roots, there was a fair chance underground retreat wasn't possible either.

Roots shot up in unison, forming a jagged wall in my path. Without slowing, I planted a foot against the nearest root and launched myself over the barrier, twisting mid-air to slice through a smaller vine that tried to ensnare me.

I kept slicing and slicing as I backed up more and more. Eventually I bumped into a tree. I looked back to see that I had already reached the edge of the barrier. I look forward, seeing multiple hands come up to squish me.

I put my hand on the tree as I prop up my Katana. The best thing to do in this scenario is to escape back into my world and come back after getting healed with proper support items.

However, just as I was about to leave, my bloodied hand grazed the tree behind.

A shining glow erupted from the tree, blinding me and forcing the roots backwards, even burning some of them. I looked back as I noticed the blood on the tree. Moving my hand to touch the blood, I notice the unusual hardness of the tree.

"Wait, this tree, it's the metal tree I encountered before"

But that's not possible, that tree was further ahead at the beginning of the forest, and I was sure I did not somehow circle back to that area considering I had been travelling in a linear path. That only means one thing, there were multiple such trees located around the forest.

A sound comes from the tree, as it expands in height and breadth. The bark disappears only to reveal a dark gate which contained a path leading inside the tree, possibly downwards underground. I contemplate the path, but looking back and seeing the forest, I realize I don't have much of a choice in this matter.

Taking a deep breath I enter the path, the tree behind me closing.