The Multiverse Voyager

Once the gates closed, I turned around putting my eras on the closed tree. The sounds of rustling outside grew quiet and then ceased to exist. I stepped back from the gate and turned back around.

My eyes took a few moments to adjust to the darkness but were suddenly assaulted by light when a few torches on the wall suddenly lit up, illuminating a stairway. The stairs twisted downward endlessly, making me unable to see the bottom. But most importantly-

"It's hot"

The air itself seemed alive, pulsating with unbearable warmth that seeped into my skin. My breath caught in my throat making me cough. Even something simple like breathing felt like dragging sandpaper through my lungs.

Within moments, my skin began to dry out, tightening uncomfortably. I ran a nail across my arms only to leave behind a white line—skin so parched that it marked with the lightest touch.

My throat ached with dryness, as though the heat had already evaporated every last drop of moisture from me. I swallowed trying to moisturize my throat, but my saliva felt thick and gummy, already evaporating before it could do anything to soothe me.

Even my lips were cracking, the thin skin peeling back painfully with every movement of my mouth. I tried to lick them, but my tongue was dry too, useless against the relentless, searing atmosphere.

It was as if the very air around me was draining the life out of me, leeching away every drop of water. I tightened my grip on my katana and pressed forward, ignoring the voice in my head screaming for me to turn back.

I approached the stairway as I began to descend the stairs, but immediately hissed back in pain from the heat of the stairs. From what it looks like, whatever materials the stairs were made of had already been burned and charred. I ran my hands across the walls, feeling the same amount of heat. Everything was scorched black.

My instincts screamed at me to turn back, to leave this place and face the demon controlling the forest instead. That would be easier—less suicidal, at least. The forest demon was dangerous, yes, but this presence… it was suffocating. The very ground seemed to tremble faintly under its oppressive weight.

"Fuck," I muttered under my breath, wiping sweat from my brow, though it was pointless—more replaced it instantly. "This thing is definitely as strong as an Upper Moon."

I glanced at the gate behind me, still sealed tight. Logically, the smart move would be to leave this world entirely, get supplies, prepare myself properly, and then come back. But that was logic. Right now, logic wasn't what was pushing me forward.

No, it was something deeper—some mix of stubborn pride and the sheer desire to test myself. To prove that I could survive, no matter how desperate the odds. If I can't even survive a world like Demon Slayer, how would I survive more dangerous worlds filled with far stronger entities.

I tightened my grip on my katana, the familiar weight grounding me. 

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

I looked at the stairs as I jumped. The stairs were too hot to traverse properly, so my only choice was to parkour off the walls, maintaining only a split second of contact with them so as not to burn my feet.

The walls weren't much cooler, but they offered less direct heat than the glowing stairs. Each contact was fleeting, a quick push-off before the unbearable heat could burn through my strength.

The narrow stairwell amplified the heat, the air so thick and hot that it felt like I was jumping through an invisible swamp of fire. My body screamed in protest with every movement, even the cloth of my uniform began to feel like an additional burden, clinging to my skin uncomfortably.

*BOOM*

With a loud sound, I finally slammed down on the ground. The heat here was even more intense, almost unbearable as if I had walked straight into the core of a volcano.

And there, at the center of the chamber, stood a massive gate, carved from burnt metal. Veins of molten gold ran through it, pulsating faintly like a heartbeat. The oppressive aura seemed to emanate from beyond the gate, making my hair stand on end.

Who would need such a thing deep underground?

I approached cautiously, my eyes scanning the edges for any signs of traps or mechanisms. As I neared, I noticed the heavy chains binding the gate, each link thicker than my arm. They were charred, yet unbroken, as if they'd been forged to withstand the immense heat that had scorched the metal.

A low rumble echoed from behind the gate, a sound that wasn't quite a growl but not quite a sigh either. It was the sound of something ancient, something dangerous. My instincts screamed at me to turn back, but I pressed on.

I approached the wall trying to knock, but the gate began to open on its own before I could do so. The walls began to widen, opening into a corridor that stretched endlessly, but was stopped by the chains.

With a loud groan, the chains shifted. I froze as one of the links shattered into ash, crumbling away. The gate creaked open just enough for me to slip through. Immediately a huge amount of heat hit me in the face, melting some of my clothes.

I endured, as I entered the chamber. Inside, the chamber beyond was vast and suffocating. The heat was unbearable, yet the air was dry as dust. A figure bound by chains sat at the center of the room, amidst a pile of blackened stone and ash.

The demon was massive, easily twice my size, his broad shoulders hunched as if carrying the weight of centuries. His skin glowed faintly like embers, and a mane of fiery hair cascaded down his back singed at the tips. His eyes opened as I approached, glowing like molten gold, locking onto mine with a piercing intensity.

"You..." he said, his tone containing an unsettling mix of curiosity and weariness. His eyes scanned my body, his gaze briefly stopping at my Nichrin blade. "How long has it been since I've seen another soul?"

I tried to swallow but coughed instead, dust filling my throat as my dried-up body began to protest against the unbearable heat. My vision blurred slightly as nausea crept in, and I fell to the ground, my hand trembling as I prepared to activate my ability and escape.

Just as the thought of retreat solidified in my mind, the demon spoke again, his voice calmer.

"Ah, my apologies."

Symbols etched into the ground suddenly flared to life, glowing with light. A burst of cold air erupted from them, The stark difference left me gasping for air, my parched throat finally finding relief as the moisture in the air revived the production of saliva in my mouth.

The mending of my clothes caught my attention next—another symbol lighting up as the melted fabric restructured itself, good as new. I blinked in astonishment, my breaths still uneven, and turned my gaze toward the demon before me.

He was immense, his frame towering even in his kneeling position. Chains, glowing faintly as if imbued with some form of enchantment, bound him to the ground. His head hung low, his expression unreadable but heavy with exhaustion. Despite his imprisonment, there was no malice in his aura, just an overwhelming sense of weariness.

"Is this your Blood Demon Art?" I asked, my voice hoarse but steady as I gestured toward the restored air and my repaired clothing.

The demon chuckled, a low, almost amused sound. "You can say that."

He leaned back slightly, resting his head against the cavern wall, his movements slow and deliberate, as though burdened by the very act of existing. The lack of resistance in his posture struck me as odd. He wasn't struggling against his chains. He wasn't raging or plotting, as most demons would in his position.

No, he just sat there, as if tired of living.

But there was something else—something unsettlingly human in his demeanor. I studied him carefully, and my gaze eventually settled on his face, more specifically, his eyes.

"Your eyes..." I said, narrowing my gaze.

He tilted his head slightly, almost amused. "My eyes? Is there something wrong with them?"

"No," I replied, the realization hitting me like a ton of bricks. "No, there's nothing wrong with them."

And that was the problem.

His eyes were clear—completely ordinary, not contained by the kanji that signified a connection to Muzan Kibutsuji or the Twelve Kizuki. There were no crossed-out kanji either, no sign that he had been stripped of a rank.

This demon, with an aura that rivaled or even surpassed the Upper Moons, had no apparent ties to Muzan.

"Are you an Upper Moon?" I asked, my voice laced with disbelief.

The demon tilted his head back, his expression shifting to one of mild confusion. "Upper Moon? What is that?"

"You're kidding," I muttered, taking a cautious step closer, my instincts on high alert. "Don't tell me you've never heard of the Twelve Kizuki before."

The demon's brow furrowed slightly, his gaze meeting mine, and for the first time, I saw a flicker of genuine curiosity in his expression. "Twelve Kizuki? No… I've never heard of such a thing."

My blood ran cold. A demon of his strength, existing outside of Muzan's reach or knowledge? That should have been impossible. Yet here he was, sitting chained in a cavern, an enigma wrapped in layers of mystery and power.

No, this could be another trick. The demon outside tricked me and trapped me in his cage, and this demon could be doing the same. No, what if this person is the demon controlling the forest, making me come underground to block off all my paths of exit?

"Do it" The demon said bringing me out of my thoughts.

"Do what?"

"The Nichrin blade," The demon said "You're a Demon Slayer right? And from your breathing pattern…Water? But that's weird, you've mastered Total Concentration Constant, something only high-ranking slayers can master, yet you are inexperienced to even deal with a weak demon like the one controlling this forest, and the Nichrin blade, it's not yours either. Are you…a trainee slayer?"

I blinked. For someone not aware of the twelve Kizuki this demon was surprisingly knowledgeable about breathing styles and the ranks of the Demon Slayer Corps. Was this demon lying before? But…why expose this knowledge knowing it would uncover his lie, and how did the demon know about my breathing technique and Nichrin blade by just looking at it?

"Again," I said, raising my blade "Do what?"

"Kill me of course," The demon said "That is your job, to kill demons isn't it?"

"I'm not buying it" I said narrowing my eyes "First there's a metal tree behind me that conveniently activated just as I was about to die, then a demon that conviniently also wants to die? What's your plan? Are you going to kill me with those chains as soon as I get close to you? And those symbols, is that how you were controlling the forest?"

"Again," The demon said "I am not the demon controlling the forest, that is another demon, and I truly wish for you to kill me. I don't know how to make you believe, but I promise on my pride as a swordsman that I will not harm you if you attempt to slay me"

"Alright then," I said as I mulled over his words. While I was unsure that he was not going to kill me, I could at least believe the fact that there was another demon controlling the forest. "And my breathing technique, how did you know that and the ranks of the Demon Slayer Corps?"

The demon's lips curled into a faint smile, revealing sharp teeth that glinted in the faint light. "I was a Hashira once. But that was... centuries ago. In the Sengoku era."

My breath caught. A demon who had been a Hashira? Impossible.

He stood slowly, towering over me, the remnants of ash falling from his body. His presence was overwhelming, the heat radiating from him almost unbearable.

"I was once the Flame Hashira," he said, his voice steady and filled with a strange pride. "Kazuhiro Rengoku. But that man is long dead. What stands before you now is a shadow—a demon who refused to feast on the living. A monster who imprisoned himself... to keep others safe."

"Imprisoned…" I muttered."You willing imprisoned yourself to not eat humans? But the Sengoku era was centuries ago, there was no way you survived without eating humans that long, you should have starved a long time ago"

"Centuries?" The demon questioned "So it's been that long huh? As for surviving? My Blood Demon Art lets me absorb any form of energy to supply my body. For some reason, my body absorbs heat as well. I cannot die from starvation"

"Thermal Energy?"

"What is Thermal Energy?"

"Nevermind"

The demon then turned his eyes to me "But for a mere trainee to achieve Total Concentration Constant? Has the strength of the Demon Slayer Corps increased this much? Even mere trainees can now achieve a technique only used by the Hashira in the Sengoku Era"

"I, no no," I said shaking my head "It's still the same, only the Hashira and their disciples, called the Tsugoku, can use Total Concentration Constant. I just sort of picked it up on accident"

"On accident?" the demon muttered in disbelief "You just…accidentally learned it?"

"Well, I have been training to achieve it for the past week" I stammered "So, not accidentally I guess?"

"A week," the demon said incredulously, his face baring the smallest hint of surprise "That's it? That's how long it took you? And I saw your strength up there, how you effortlessly ripped out large chunks of the ground, are you truly human?"

"I am," I said " And, contrary to what you said. The strength of the Demon Slayer Corps has decreased since the Sengoku Era. The Hashira of this generation possess neither the Demon Slayer Mark nor the See Through World"

"Truly?" the demon said in surprise "And they still call themselves Hashira?"

Kazuhiro sighed as he shook his head "Though, I suppose that is at least good news. If the strength of the Slayer Corps has weakened then that means times have become more peaceful compared to the Sengoku Era"

"Peaceful yes" I nodded "After the Demon King got his ass handed to him by Yoriichi Tsukiguni he's stopped roaming around and causing mayhem and now hides himself among humans trying to find the blue spider lily"

"Hide?" Kazuhiro said mockingly "I had no idea the Demon King who possesses enough strength to effortlessly defeat the Hashira, and I mean actual Hashira, not the posers of this era, was so cowardly"

"Yeah," I said "That's why he created a group containing twelve elite demons, calling them the twelve Kizuki. They're divided into two groups, the lower moons, and the upper moons. The lower moons are manageable, but the upper moons are as powerful as Hashira with a Demon Slayer Mark. The three strongest Upper Moons cannot be defeated by a single Hashira even if they possess a Demon Slayer Mark, you will need multiple Hashira to do so."

'Of course, there are exceptions, like Yoriichi or Gyomei'

I pointed to the demon's eyes "Lower moons have a number in one eye, and upper moons have a number in both eyes. If a person was formerly a moon but got kicked out, the number in their eyes is crossed out. That's why I was surprised to see nothing in your eyes"

"I see", Kazuhiro said, furrowing his eyebrows "These Upper Moons, they must have been alive for centuries eating people and gaining power haven't they?"

Kazuhiro gritted his teeth "What have the Slayer Corps been doing? The Upper Moons weren't always strong, how did they let demons get this powerful? Have the Slayer Corps weakened to this point, or have they just become incompetent?"

"I can't say," I said, shrugging. I haven't even joined yet, but there are thousands of demons roaming. You can't expect them to be able to hunt a specific demon, down, right?"

"Don't make excuses for the Corp's incompetence" Kazuhiro cut me off sharply "Do you know how many people these demons have killed? How many people they have eaten? Is this what you will tell their families, that we were too busy killing other weaker demons?"

The chains began to groan and creak as an oppressive pressure rose from Kazuhiro. "The role of the Hashira is to take on the strongest demons, they are not supposed to kill baby demons or help incompetent slayers. If you can't kill demons then don't join the slayers or accept death with honor."

Heat began to emanate from Kazuhiro as the symbols below glowed brightly. The symbols produced more cold air but failed to make a difference in the heated atmosphere. The familiar feeling of dryness appeared in my throat as I coughed.

The chains began to heat up as they started to glow.

"My blood boils with rage when I think of an innocent life that could have been saved. It is the responsibility of the strong to protect the weak, that is why we have our gifts.

Hashira? If mere weaklings can be crowned the title of Hashira then it is no wonder the demons are growing stronger. We had to fight with our lives, train to the extreme to the point where our bodies failed us, sacrificed our lifespans, and destroyed our vision to achieve the pinnacle of breathing techniques, and then we were called Hashira."

The chains melted as they snapped. The heat slammed into me, forcing me to bring my arms up to my face and close my eyes.

"I was content with toiling my life away here, I trusted the next generation, but they failed."

The chains finally snapped, falling to the ground with a loud thud. Kazuhiro rose to a height of almost 9 feet. Markings of flame engraved on his face began to glow with an orange hue as he stepped out of his "prison".

"But I will not fail"

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Ight so, not many of y'all know about the Sengoku Era in Demon Slayer, not sure the anime has got that covered yet, so if you want to know, spoilers below.

The Slayer Corps didn't always have breathing techniques. When they started out, they just had normal samurai or swordsmen. During the Sengoku Era, Yoriichi Tsukiguni taught the Demon Slayers breathing techniques. The Hashira rank was only created after breathing techniques were introduced

The Hashira of the Sengoku Era were the strongest generation of Hashira, each having the Demon Slayer Mark and See Through World. Even ranks below the Hashira could use Total Concentration Constant and the slayer mark. Not just the slayers, but the blacksmithing techniques were also at their peak during this era.

Combine this with the fact that the twelve moons were not even created at that time, this was pretty much the golden age of the Demon Slayers.