"Wow okay. Uh, Haroun, I'm gonna do what the slime gang wants, they have said they mean to compensate you for your fright, I will make some reasonable amount of attempt to meet up with you at the inn if that still makes sense after I have heard out the emptiness dragon thing but if this turns out to take more than a day or two maybe go to Cowcorn without me and I will meet you there if that still makes sense and otherwise I won't?"

"I... obey..."  Haroun doesn't know what is happening.  He's never heard of anything like this happening to anyone.  All he can do is whatever the Summoned Hero says.

<Companion compensated - with knowledge - training - in lost art.  He will exit dungeon - when you do.>

Fortunately Haroun is already carrying all the loot and Bella is already carrying all of the artifacts from a higher world she wants to maintain personal control over. "- never mind they are going to train you in a lost art and you will exit the dungeon when I do apparently I'm really sorry about all this I hope the lost art is great." She is unsure how long the slimes are going to put up with this chatter and talking kind of fast.

The slimes assemble by the entrance to the room that they entered from.  Waiting.

She steps slimesward.

The journey that follows is shorter than it should be.  Unexceptional sections of wall part, revealing unlighted tunnels behind; a couple of the Slimes begin glowing to light it for Bella's convenience.  The mighty doors of the first floor boss room open, and the enormous tree revealed there stays silent and quivering as the Slimes escort Bella past it.  Not to the sealed door that lies beyond the great tree, but to a section of wall nearby that also slides aside, revealing unlighted stairs.

So it goes for a time.  Hidden tunnels, normal rooms with monsters that freeze up and quiver as Bella passes, hidden staircases in or near boss rooms.  Some stairs descend further than others, going two or three floors down at a time.  Now and then Bella hears distant voices; Adventurers talking among themselves.  She could, if she wished, raise her voices and call out to them; perhaps the Adventurers on lower levels would be strong enough to defeat her escorts.  Haroun is still up there, of course.  Does she call out?

No, she's not going to drag third parties into this. So far nobody's been hurt and it doesn't seem like it'll stay that way if she starts yelling 'hey, professional slaughterer of pineapple babies and suchlike, please involve yourself'.

In time a dark passageway terminates before great stone doors bound in gold.  The doors aren't very impressive for a Final Boss fight by the standards of larger dungeons, but Bella is unlikely to have personally been inside any dungeons with doors more impressive than that.

The doors open, and the Slimes escort her in, revealing a great dark chamber.  A lone torch lights at the center of the room, as she enters, revealing a raised black marble plinth at the room's center.  The plinth is empty.  In the lone torch's light it is barely possible for Bella's dark-adjusted eyes to see that there are smaller golden doors at the other side of the room.

The Mutant Chameleon Metallic Slime slurps its way onto the raised plinth, resuming its place as the Final Boss of the dungeon.  It then stands there, motionless and quivering; and slowly turns translucent, revealing a brighter core inside itself.

It raises a tentacle and reforms it into hands, one final time.

<You must slay - the final boss - for the final doors to open.  There is no other way.  Cleave the slime's core.  It will revive in time.>

"...hell. Okay, I'll..." She doesn't want to level up in swords, and if she's going to slash through a boss monster she might as well get the XP counting toward something else in addition. "Eval Flash," she mutters, and then she gives it a slice.

The sword cuts through the slime's core as easily as the sword has cut everything else.  The Mutant Chameleon Metallic Slime falls apart silently into goo, and dissipates into smoke, leaving behind a shimmering cloak.

Bella feels stronger, in some way that's hard to pin down.  The fatigue from casting Flash has faded more quickly than before; she's sure of that now.  Her sword rests steadier in her hand, in her improvised stance.

The golden doors at the back of the room swing open.

So is she supposed to - take the cloak? - yeah, the boss monsters' respawn mechanics can't reasonably depend on adventurers not taking everything not nailed down. She takes the cloak. She goes through the doors.

You are in a chamber of white marble, well-lighted by the soft glow of the walls.  In the room's center, a stand of glowing marble rises from the floor.  On it is a single key of crystal, one key and only one key.  To your front and left is a barrier of transparent crystal, behind which lies a lesser chamber of treasures: you can clearly see a small scattered heap of gold coins, daggers and gauntlets not made of tin, rings with glowing gems.  To your front and right is another barrier of transparent crystal, beyond which lies a small stone room empty but for a hovering one-meter-diameter orb, glowing with an inner red light and surrounded by orbiting ribbons of tiny runes like a planet's rings.  Both barriers of crystal bear openings into which the key on the stand might fit.

There is one crystal key on a marble stand here.
There is a locked crystal barrier to your left.
There is a locked crystal barrier to your right.

Keys are reusFUCK keys might not be reusable in Stupid RPG World. Okay. Neither of these things are a void dragon. The cool planety thing is cooler. She has no idea what its resale value might be or what she could possibly do with it besides get a closer look, though. Like, it seems like it might be dumb to touch it. It... might also be dumb to touch the treasure? Maybe it's fake trap treasure? Where is the void dragon. Oh hey maybe the orb is the dungeon core. She takes the key to go visit the orb.

The key and the crystal barrier both melt away, allowing Bella to step into the stony room.

As she enters she can feel a tangible sense of power in the air, a crushing force bearing down, laced with a sense of compulsion.  It centers not on Bella (for that indeed would be futile) but on the dominated dungeon orb.  Glowing letters are forced to spin out from it, forming and empowering a dome around Bella.  She can read the runes that light up in the air, but the expressions they form are too alien to comprehend; she can only pick up a vague sense from the entire construct.

Space
Boundary
Symmetric
Light
Sound
Thought

- form a spatial boundary which will symmetrically pass vision, sound, and thought -

There is a great flash that Bella can feel, and when it subsides, Bella is apparently elsewhere but for a faint soap-bubble of slight distortion that surrounds her, saving only a ring of the original stony floor about Bella's feet.

If you ignore the soap bubble of distortion and trust what your eyes are telling you, you are standing in the middle of a skyscraper-sized chamber of black stone, facing a city-block-sized silhouette of a Dragon.  The Dragon's silhouette is pure darkness, vantablack, fuligin, the unremediated absence of vision.  This silhouette of pure darkness, as near as you can make out, shows the shape of a Dragon coiled and its head rearing to face you.  The Dragon's silhouette rests on a heap larger than itself, a heap of gold things, platinum things, things of red metal and white metal and pure radiance.  In the parts closest to you, you can make out the tiny shapes of armors and arms, greatswords and helmets, set with many jewels and runes of 『Language』.

There is a silhouette of a Dragon 'here'.
There is an awful lot of treasure 'here'.

The silhouette of a Dragon's head opens its mouth, and speaks.

"I'm still getting - oriented? I should have a more concrete answer in a week or two. Uh, I favor the general flourishing of all sapient beings." She fumbles for her notebook, flips to a page, starts jotting down key phrases as they go by.

A huge laughter, larger than the chamber in which it echoes.

"- wait, what? That wasn't presented as an option I had!"

"Then wouldn't the hero and the demon lord be in symmetrical positions? Why would there consistently be one of each?"

"Have you seen signs of them?"