Tiona, whose Falna included the [Diving] development ability, led the charge and plunged into the lake.
Atop the cylindrical tower.
Clearly visible, set into the floor of blue-grey stone — there was a black door.
Its presence was impossible to ignore. In the midst of this limpid azure water, the mere fact that it existed there drew the eye irresistibly.
Beyond that door.
A "terrifying world" awaited, most likely.
There was no time to hesitate, no time to think. Tiona was the first to pull it open and step inside.
Isagi directed Teardrop Big Bro to cover the rear, then had Ryuu-senpai take Ais down with her. The girl who couldn't swim at all had barely opened her mouth to protest before she was yanked into the water, thrashing on pure instinct.
Fortunately, the tower wasn't far below the surface. Ryuu-senpai got the golden-haired girl through the door in short order.
She confirmed there was no danger on the other side.
After that, Lefiya, Riveria, Haruhime, Goddess Artemis, and Saika passed through in sequence. Last came Isagi himself and Miss Heith, serving as rearguard.
The moment he crossed that pitch-black threshold —
Isagi heard it again: a crisp, clear pop beside his ear, like a bubble bursting.
As if he had broken through a water's surface, he was suddenly, entirely, somewhere else.
The black door swung shut on its own.
Isagi was still for a moment, registering the cold, hard sensation against his back, then slowly rose to his feet.
The magic-stone lantern at his waist cast a warm amber glow.
Tiona, who had come in first, was holding her own lantern aloft, trying to push the light as far and wide as it would reach.
The space around them wasn't pitch-black.
There were stones — blue, like fragments of crushed crystal — embedded throughout, pulsing with a faint luminescence that lit up vast stretches of rocky wall.
Everything was damp. That made it bitterly cold.
Looking up, straining his eyes, Isagi could just barely make out the ceiling far above — and set into it, the black wooden door they had come through.
Strange.
Had they really fallen from that height? He'd felt nothing at all.
Something was off, though he couldn't quite put his finger on what.
At the very least, this underwater structure wasn't flooded with water, which made Ais breathe a small sigh of relief. She had apparently been bracing herself for an entire adventure conducted underwater.
That would have been nothing short of cruel for someone who couldn't swim a stroke.
For now, what lay before them looked simply like a vast cavern — not so different from the Dungeon they all knew.
The only problem was —
"This place is massive, don't you think?"
Ryuu-senpai and Riveria both felt it the moment they looked around: something was wrong.
The lake above hadn't been small, and the underwater castle it contained was enormous, but the tower they had entered was only a tiny corner of it.
"Can't be more than a few dozen meters across, surely?"
"Definitely smaller than Babel."
"But the space in here is bigger than Central Park."
The girls murmured to one another, all of them having noticed the same impossibility.
And then.
Saika — the small jellyfish drifting through the air nearby — let out an adorable little yelp.
"Ow!"
The little creature had clearly bumped into something.
Moments later, Isagi noticed the problem himself: there appeared to be invisible walls all around them.
The cavern looked impossibly vast — spacious enough to hold tens of thousands of people without complaint.
Yet when Isagi drew his greatsword and probed to one side, the blade stopped dead. Just a few meters away, something unseen blocked its path entirely.
The girls discovered it too, quickly enough.
They realized that what they were actually standing in was not the soaring, cavernous space they could see — but what should have been a corridor or passage, roughly three to four meters wide and no more than five meters tall.
"What's going on? What do we do?"
Tiona frowned.
If it was a passage, then their only options were to go forward or go back.
But then — what were these invisible walls? Was there a dead end ahead? Where did this passage even lead? And —
"Stop!"
Isagi, who had been walking at the front, halted abruptly. His right boot, clad in black iron, had sunk into the wet ground — and the ground wasn't there.
He had misjudged his footing.
His foot was dangling in empty air. There was nothing solid beneath it — nothing real at all.
Not just invisible walls, but invisible pits, too?
What was down there?
With no way to use their eyes to judge the danger, everyone felt a fierce, involuntary surge of dread.
But the truly baffling things were only just beginning.
Monsters appeared.
Just ahead of the group, exactly as if following the Dungeon's ordinary monster-spawn cycle — first a massive crab hauled itself up out of the floor, condensed into form, and came charging at them, great claws brandished.
This was a Blue Crab, a common monster of the [Great Waterfall] area. On their way down here, the group had already killed more of them than they could count.
Their standard tactics: slam downward with those twin claws, and fire jets of water from their mouths for ranged attacks. A well-rounded threat — strong on defense, effective at any distance.
But.
When those Blue Crabs came swarming toward the group — closing the distance without a stumble, without hesitation —
Isagi noticed: they hadn't fallen into the pit at all. They walked straight across the space where the ground should not have been.
"?"
His own foot was still hanging in empty air.
Was the pit too narrow for a crab to fall through?
Or was it something else entirely —
A Blue Crab's claw came crashing down.
In that instant, on pure combat instinct and muscle memory, Isagi didn't reach for the Moonlight Greatsword — he drew the Bone-Heap Blade instead.
A golden flash erupted with a sharp, violent screech of metal on metal.
A block — successful!
But what he had blocked wasn't the crab's claw. It was something sharp, something unseen, a thrust aimed straight at his heart.
A completely invisible attack!!
A moment later, the crab's claw was sheared clean off — Ryuu-senpai's work.
Crimson flames wreathed a scattering of cherry blossoms.
A slash from her spell-blade magic split the monster before them into pieces in an instant. Then Ais and Isagi moved together, cutting down every creature bearing down on the group in the span of a breath.
Among the monsters they faced, the Blue Crabs were familiar enough that veteran adventurers like Ais knew exactly how to use their weapons to "dig" the Magic Stone straight out of a crab's body.
The moment the Magic Stone left the body, the massive monster dissolved into ash and vanished.
Then came the sound of Magic Stones striking the ground.
"Two of them!"
Goddess Artemis's ears missed nothing.
The hunting goddess caught it instantly: two Magic Stones had hit the ground at the same time.
Which meant that in addition to the visible Blue Crab, there had been a second, invisible monster attacking them simultaneously.
Were they overlapping in the same space?
Or was it something else —
Ais used her flight ability to probe the invisible pit blocking their path. It was roughly five to six meters wide; how deep it went was beyond anyone's ability to test.
The far side had solid ground. For adventurers, the obvious answer was to jump.
Haruhime and Goddess Artemis, whose physical abilities were less suited for that, were carried across by Isagi and Ais respectively — both of whom could fly.
But this was only the beginning.
As they pressed on, Isagi and the girls kept finding more things that made no sense.
The path beneath their feet began to slope downward.
They walked, and walked.
And after a long while, the lower halves of their bodies had visibly sunk below the level of the ground — which was plain to see.
What made it worse was that monsters attacked them from below as well.
Snakes, perhaps. Or some kind of insect.
Isagi and the girls could only rely on fighting instinct to react to incoming blows — blows they couldn't predict, couldn't see coming — and in the chaos, they couldn't even see their own hands, let alone the arc of their swings.
And then the thing that gave Isagi the biggest headache appeared.
Further ahead, a cluster of ranged-attack monsters had spawned.
Crystal Creatures.
They fired magical light-attribute artillery — among the most troublesome monsters in the [Great Waterfall] area.
Thankfully, there weren't many of them.
In numbers, their light-attribute magic could refract off one another and amplify, growing devastatingly powerful.
But that wasn't the problem. The problem was what was hidden inside the light — a ranged attack no one could see.
Was it magic?
Arrows?
Something else entirely?
There was no way to know. All Isagi and the girls could feel was that something was being launched at them.
Alongside the blinding white-light cannonade, that unidentifiable, unknowable ranged attack bore down on them all at once.
Fortunately, Lefiya and Riveria were both acutely sensitive to danger, and the two mages had already begun chanting defensive spells the moment the Crystal Creatures appeared.
"Raise the cup as a shield — chalice that breaks all evil!"
A deep-blue curtain of light materialized in front of the party, blocking every attack — visible and invisible alike.
Then:
"——Gather, breath of the great earth — I am Alf!"
"Gossamer Breath."
A pale green luminescence spread across everyone, settling into a faint, shimmering barrier.
This was Riveria's second-stage defensive magic — a form of enhancement spell that raised physical and magical resistance for every adventurer it touched, deflected a certain degree of incoming attacks, and carried a slow but steady healing effect.
For now, it had held off the sudden, unseen ranged onslaught.
"This is so wrong. This is so, so wrong," Ryuu-senpai muttered.
She was not the only one bewildered by what the Dungeon was doing. Even Isagi was baffled, wondering what new trick the Dungeon had pulled out this time.
He had no idea either.
They kept moving. And at some point — in an instant, or perhaps just a second before — everything changed.
All at once.
The vast cavern they had been walking through was simply gone.
In its place was what looked like the interior corridor of some castle. Stone walls rose on either side, not far off, equally damp and dripping, radiating a bone-deep chill.
Beneath their feet, the passage continued to slope downward.
The ceiling soared impossibly high — too high to see the end — and from somewhere above it, a faint pinkish-red glow spilled down, illuminating everything around them.
This place was —
The ground beneath his boots felt exactly as solid as before. Nothing had changed in that regard.
Apparently, from the very beginning, they had been walking along this slope — and now, all that had changed was that they could finally see it.
Had the Dungeon gone back to normal? Or was it...
They pressed on, carefully.
And encountered the exact same problems as before.
Monsters spawning without warning and attacking. Invisible assaults that couldn't be seen or heard.
And then, mid-walk, invisible walls materialized ahead of them again.
The slope clearly kept going down.
Yet the way forward was simply blocked. Isagi felt around and found that the left-hand wall could actually be passed through. After a brief stretch of darkness, the group emerged into a room somewhere inside the castle.
It wasn't wide. The walls and floor were still soaking wet all around.
The ceiling was the same impossibly high expanse, radiating that faint red glow. The room itself looked clear at a glance — unobstructed, open.
And yet Isagi and the girls walked straight into walls every few steps, forced to change direction, as if the space before them was in fact a labyrinth of staggering complexity.
What was happening?
Were spaces overlapping?
Or did several entirely different scenes coexist simultaneously — and were Isagi and the girls always materializing inside "the other scene" (the one they couldn't see)?
——And at the same time, whenever monsters spawned, each different scene spawned its own monsters, all of which then converged and attacked the group at once?
A new trick from the Dungeon — wait. No!
Something clicked for Isagi, and he recognized an oddly familiar sensation.
This couldn't be the [Inside-Outside World] — his personal favorite nightmare — could it?!
And on top of that, the overlapping worlds were all completely different from one another, which meant you had to shift between them to find the path forward.
He said nothing.
Isagi simply kept moving, one careful step at a time.
Instinct told him that whatever place the Dungeon had spent so much effort preparing, it was not going to be this straightforward.
In all likelihood, something far worse was waiting up ahead.
Isagi had once thought that his arrival in this world must have affected the Dungeon — that it would have diverted some of the power it was using to gestate the [Black Dragon] in order to deal with him.
Turn part of that energy into a "special Dungeon."
But thinking it through now —
That wasn't necessarily a good thing at all!
The Dungeon as it stood was already in the process of gestating something as terrifying as the [Black Dragon]. Even if it redirected only a fraction of that power against him — could he actually handle that?!
After all, the welcoming gift at the very start had been a Skeleton King BOSS of at least Lv. 6!
And if it was true that the deeper he went, the more of the Dungeon's attention — and power — would be turned against him...
Surely it wouldn't —
Surely the [Black Dragon] wouldn't fail to gestate entirely because of him, leaving the Dungeon with nothing to do but pour every last bit of its strength into killing him instead?!
For some reason, the moment that thought crossed his mind, even Isagi found it darkly absurd.
So — was he actually going to cause the Dungeon to have a miscarriage? Was the [Black Dragon] just going to... not be born?!
And what would take its place?
Isagi didn't dare think about it anymore.
____
👻🔥Walnut-chan🔥👻
🔥 New history: Oshi No Ko: Co-starring with Kana Arima
🎯 100 Powerstones = +1 Bonus Chapter for everyone