Chapter 1
It was a beautiful day outside: birds were chirping,
flowers were blooming. Apparently, days like these are perfect for isekai.
I was sitting in my Tuesday afternoon history class. The room was dusty and looked like it had seen most of history itself, judging by all the cracked and broken relics scattered around. My overexcited history sub, Miss Aiko, was in the middle of another dramatic rant about some ancient war when, suddenly, a blinding white light engulfed the room…
That was when my story – the story of the introverted Yuuno – truly began.
My eyes snapped open.
The classroom was gone.
Instead, I was in a rough stone cave speckled with moss and glow‑worms. I tried to look to the right. Instead of just my eyes moving, my entire field of vision slid sideways, as if I’d turned my whole body ninety degrees without actually moving.
I instantly wondered where my classmates were…
“Welcome, user Yuuno,” said a sudden, synthesised female voice inside my head.
“Uuuuuuum, hi?” I answered, my voice shaking somewhere between terror and curiosity. “Who is this, exactly?”
“I am your Dungeon Master…”
I turned – or whatever counted as turning – toward a nearby puddle. My reflection stared back at me.
It wasn’t human.
A floating blue crystal hovered over the water, glowing faintly.
“And what am I?” I asked carefully.
“You are a dungeon core,” it replied simply. “The operator of the dungeon.”
Before I could panic properly, a black screen appeared in front of me, covered in white text. It showed my stats, a shop, and how many PT I had.
My Dungeon Master Intelligence – the DMI – ran me through the basics: once I hit level 10, about average for a dungeon, I’d start burning 1 PT an hour just to keep running. I would earn PT by killing adventurers, then spend those points on functions, traps, monsters, and evolutions.
Right now, I had 1,000 PT – starter money for a starter dungeon.
I grabbed some basics: a handful of starter goblins and slimes, plus an auto‑respawn function for my monsters. My screen showed a rough layout of my dungeon: a little over a dozen rooms and a few traps.
I spread monsters through the rooms, wired in the traps, and slapped respawn on anything that moved.
As time passed, I noticed I was earning a slow trickle of PT from the monsters themselves. Thin streams of mana flowed from them into my walls, feeding me. After setting everything up, I was down to 200 PT – but the extra mana from my monsters soon boosted that back up.
That was when a lone adventurer wandered in through my entrance.
I watched her carefully. She moved like she knew what she was doing. I could have killed her easily with a trap… but I held back. Instead, I let her leave, figuring she’d spread the word and bring more people here.
She ran off – obviously to report a new starter‑level dungeon.
“She was high level,” the DMI informed me calmly. “Defeating her would have rewarded you with at least 150 PT.”
I mentally facepalmed and scolded myself for a hot minute for passing up free points.
Two days of passive PT collection later, I was back over 1,000 PT.
In theory, I could have kept sitting there, letting my goblins and slimes trickle mana into my walls forever. In practice, my monsters were idiots.
Case in point: the next party that came in.
This time, a full group of weak‑looking starter adventurers: two sword guys in mismatched armour, a nervous girl clutching a staff, someone in light leather with too many knives, and a robed healer hiding behind everyone else.
They crept forward.
The spike pit at the entrance had reset. I watched them test the floor with a spear.
“Oh, come on,” I muttered. “Just one more step.”
The spear tapped the solid stone in front of the pit. The swordsman grinned, declared it safe, and took a huge, confident stride.
Two of them plunged to their doom.
The remaining three flailed at the edge of the pit, pale and shaking. One tried to grab a friend who was falling and almost went down with them. The healer’s scream echoed off my stone walls.
A notification blinked in front of me – I’d gained 500 PT.
They didn’t run.
They pushed on anyway, hugging the walls and checking every flagstone as if it might bite them. Smart. Annoying, but smart.
Deeper inside, a squad of five goblins and two slimes ambushed them. The party barely survived the first clash; one adventurer was badly injured, and the others looked exhausted. The healer girl’s hands shook as she cast, her light barely sealing cuts before the next goblin charge.
Still, they refused to turn back.
In one of the side chambers, they stumbled into what I’d designed as a “reward” room: a cramped, square space with a single stone pedestal in the middle. On top sat a small iron chest bound with tarnished brass; dust pooled around its feet.
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“Oh, finally. Something good,” the rogue muttered, limping over. He prodded the chest with his spear first, then checked the floor for pressure plates and the lid for wires.
No traps. I’d made sure of that – I wanted them greedy.
He flipped the lid open.
Inside lay a handful of low‑grade potions, a few silver coins, and a cracked mana crystal that was still usable if you didn’t mind a mild buzzing headache.
The healer’s eyes lit up. “Healing potions!” She snatched one of the bottles and downed it, colour slowly returning to her face as the worst of her wounds closed.
“See? This dungeon isn’t all bad,” she said weakly.
I tried not to feel guilty about the arrow trap waiting in the next corridor.
They shared out the coins, tucked the remaining potions into their belts, and pushed deeper in.
Room by room, trap by trap, they were worn down. An arrow trap took the rogue in the leg. A slime dissolved half of a shield. A goblin’s rusty knife slipped past someone’s guard.
In the end, all of them died horrible, messy deaths.
I felt a stab of guilt watching the last one fall. They’d fought hard. They’d come so far. If they’d turned back earlier, maybe they would have lived.
But as their bodies cooled on my stone floors, thin streams of mana rose from them and from my monsters, seeping into the walls and into me. My PT counter ticked upward with every second.
I watched my numbers climb.
Level 2. Level 3. Level 4.
I was already level 4 from a mix of mana gathering and adventurer kills, and I needed to keep growing if I wanted to survive long‑term. The DMI’s earlier explanation echoed in my mind: once I hit level 10, I’d start burning 1 PT an hour just to exist. If I wasn’t prepared, I’d just… blink out.
So I kept going.
Over the next few days, more guild parties tried their luck.
One team spotted the entrance spike pit in time and used a plank to bridge it. I watched them smugly congratulate themselves – then dropped a ceiling net on them in the next room and let my slimes ooze in from the sides.
Another party made it to the mid‑rooms, working together almost perfectly. The tank drew aggro, the mage blasted my goblins with controlled bursts of fire, and the healer shielded everyone from arrow traps. I had to chain three rooms’ worth of monsters to bring them down.
Just before that final ambush, they found one of my “proper” treasure chambers: a circular room with a raised dais in the centre. A larger, polished chest sat there, gleaming invitingly under the glow‑worms.
“Too obvious,” the rogue muttered, squinting at it.
The tank tapped the tiles in front of the dais with the butt of his spear. Nothing. The mage muttered a detection spell and frowned.
“I’m not sensing any magic. It might actually be safe,” she said.
It was. Not every chest needed to bite.
They approached as a group this time, weapons ready. The healer hovered near the doorway, one hand already glowing with defensive magic.
The rogue eased the lid open.
Inside were a couple of decent‑quality potions, a small pile of coins, and a dull iron ring sitting on a velvet cushion. The mage picked it up, murmured another detection spell, and blinked in surprise.
“Cold‑resistance enchantment,” she said. “It’s weak, but still… that’s rare for a starter dungeon.”
“Score,” the rogue breathed, slipping the ring onto his finger. “See? All that pain is worth it.”
I let them have it. A little hope made it so much more dramatic when the goblins and slimes swarmed them three rooms later.
By the time their screams faded and the last notification blinked away, I had more PT – and a knot in whatever counted as my stomach.
“This is fine,” I told myself. “They knew what they were signing up for. Probably.”
The DMI remained silent.
While I was grinding away in my dungeon, my newly summoned classmates were just waking up elsewhere.
They’d all become humans or at least humanoids. There were twenty‑three of us in total, counting our teacher. The main ones – at least for now – were a boy called Okama, a girl called Mai, my teacher Miss Aiko, and Mai’s summon, Puss: a black cat no one realised used to be the bully girl from our class who had tormented Mai.
We’d get back to them later.
Days blurred into each other.
Parties came and went. Some ran at the first sign of trouble. Some pushed deeper than they should have. One group actually made it to what passed for my “final” room and stared around in confusion at the lack of a boss.
“I’ll get to it,” I grumbled, watching them retreat, battered and swearing revenge.
After a good day of guild parties trying to traverse my dungeon – two full wipes, one partial retreat, and one particularly embarrassing goblin incident I refused to think about – I finally reached level 10.
A surge of power rushed through my crystal. The walls hummed. New menus unfolded in front of me like a blooming flower of horrible, tempting options.
Now I actually needed 1 PT an hour just to stay awake.
The upside was that I unlocked three evolution options: more PT per kill, a bigger core and mana pool, or the power to expand my dungeon as I levelled.
I stared at the choices.
More PT per kill meant faster growth… but also meant gambling harder on a steady stream of victims.
A bigger core and mana pool sounded safe, but if I couldn’t expand, strong parties would clear me too easily.
Or I could choose the power to expand my dungeon as I levelled: more rooms, more corridors, more places to hide monsters and traps.
In the end, survival won.
I chose the third. In the long run, more space meant more rooms, more monsters, and more ways to survive.
My map display immediately stretched outward until it was at least five times bigger. Empty grey squares blossomed into potential: future lairs, mazes, and murder‑hallways just waiting to exist.
“Well done, user Yuuno,” exclaimed the DMI. “Excellent planning for the future!”
“Thanks, I guess,” I said uncertainly. “I was just thinking it would help if I didn’t have to spend PT on expanding the dungeon.”
With that in mind, I dropped a chunk of PT on a hobgoblin settlement and my first high‑level monsters – a pack of croc‑men – all with respawn. The rest went into nastier traps: more spikes, more arrows, more tripwires.
The first time a mid‑level party stumbled into the croc‑men’s swamp corridor, they laughed at my slimes.
They did not laugh for long.
By the time I was done rearranging everything, I still had 1,000 PT in reserve, enough to coast for about a month without killing anyone.
I was already level 12.
A solo adventurer appeared in my dungeon.
He walked like he owned the place – relaxed, casual, blade resting on his shoulder. My greater slimes and hobgoblins were light work for him at first, but when he finally fell in the croc‑men’s territory, I realised he’d been pretty strong.
On his way in, he’d even found one of my experimental mimic chests: a perfect copy of the plain wooden boxes I scattered around the mid‑area.
He circled it warily, then jabbed the lid with his sword. When it didn’t immediately sprout teeth, he relaxed and opened it.
The mimic was slow, still half‑finished in my system, so instead of biting his arm off, it only managed a feeble snap at his fingers before dissolving into slime.
“Weird,” he muttered, scooping out the handful of coins and a cracked mana crystal I’d actually put inside. “Buggy dungeon design.”
I made a note to upgrade my mimics later.
When he finally dropped to the croc‑men, my notification chimed.
I earned 400 PT from him alone.
Word was spreading.
My dungeon had apparently become a sort of tourist attraction, drawing high‑level adventurers from all over. I had enough PT to summon my first boss for the end of level one, but I decided to hold off.
First, level one needed to grow.
I expanded the mid‑area and summoned golems to guard it. Then I bought an element‑applier function and started assigning elements to my slimes, my golems, and even a few hobgoblins, turning them into goblin sages.
The dungeon was no longer just a cave with a few traps.
It was becoming something real.