The thieves took its mana lock.
Goblin Cave was almost impressed by their sheer avarice. Replacing it wouldn't be too costly; it had reduced the mithril down to a thin strand looping around a more mana-cheap framework of gold and mana quartz. It was more upset about the interruption to its imaging record: now it would always have a several-hour blank spot in its calendar helix.
While they had been doing it, Goblin Cave had been more than tempted to skewer them. Manipulating mana around foreign bodies was vastly more costly than doing it deep within the comfort of its own dungeon, but it wasn't prohibitively expensive. And when all the thieves were on the far side of its mana lock, it didn't even have that excess cost; it could have extruded a manacrystal and serpent obsidian lattice all across its entrance, which, in addition to powerfully restructuring mana waveforms, was also very sharp and spiky.
Still, it was going to secure its replacement lock more securely, and ring it in obsidian spines.
The thieves remained for nearly a day -- dawn to dusk -- and eventually managed to get the nerve to broach its corridor and step inside, after they'd gathered the scattered manastone on the surface. They'd used chisels to shatter the surface layer of black glass off the manastone backing, and then carved off great misshapen chunks of coarse manastone from the raw granite. Its dungeon structure was reinforced against tunneling, of course, but Goblin Cave hadn't been shaping the corridors with a mind towards durability. The manastone all but peeled away from the granite behind it, so it only took cracking the manastone in one place and widening the gap before the entire seam collapsed in on itself. They'd only left once they couldn't carry any more.
Goblin Cave waited until they left -- when the sound of their movement faded -- it constructed a fresh mana lock in the same location and resumed image recording. It slowly repaired its hallway, until it was the same immaculate square corridor it had been before the thieves had taken their picks to it. What a disappointment. Also, it should probably be concerned about the mana lock. It wasn't really important as such, just a chunk of fused metals, but it was increasingly focused on the concept of information asymmetry. There were many things it did not know about the world. There were, apparently, many things the world did not know about it. It couldn't rank dangers to itself if it didn't know what dangers existed. And the mana lock -- what all could somebody determine about it, from that?
But in truth, Goblin Cave didn't even know if they'd taken it intact: it had been its eye, after all. Given the rest of their predilections, it wouldn't have been surprised if they had shattered it to pieces and only taken the rare metals.
Still, all things considered, it would rather avoid that situation repeating. It was tempted to spawn in some higher-level mobs just behind the mana lock, to provide a little pushback -- but, of course, all that would do was attract even more adventurers. Unless the mobs could kill every one of them -- difficult, at the very entrance to its dungeon -- mobs were just experience sources.
Still, it would have to do something. If nothing else, it was still planning on restructuring its original floors at some point, and it didn't want those to be ransacked clean.
One simple change was to interlace the manastone with the granite, forming thousands of spined pins of granite affixing the manastone to it, and thousands of spined pins of manastone affixing it to the granite. Another was to add more mana locks and backup eyes. It studded cubic crystals across its entrance corridors, adding alcoves and outcroppings, and shoved half-buried mana locks and a haphazard collection of imaging tubes to the area near the entrance. It tweaked the mana locks: not cutting off all mana, since that would defeat the point, but instead attenuating it, making it flutter in a way that would disrupt many spells. This required a mana flow, so it created a loop behind the entrance, forming a primitive form of its mana bellows within it: a slowly-revolving current of mana, billowing widdershins along the circuit.
In addition, it rigged up some of its most intense mana tubes and aimed them at the miniature bellows, angled to either interfere or reinforce the mana current. It certainly wasn't a stable configuration; the mana current would sputter and die after only a few minutes. But in the mean time it could conjure up a mana whirlwind within the entryway, and hopefully if it triggered the mechanism any time somebody started gouging out its walls that would at least interrupt them for a while.
Then it focused its attention to floor 26, its enormous torus. It had needed to restructure the surface of the torus, after its initial experimentation with mana resonance proved more energetic than it had expected. Its walls were layered in ridged rings that reflected mana without refracting it, and the dense mesh of manacrystal blended with mostly mundane iron formed an intensely resilient material that was still flexible enough to meet each resonant mana pulse without shattering.
The main problem with that was that it didn't look right. Massive sheets of shimmering blue-black metal wasn't quite the look it was going for for these corridors, and it was willing to make some concessions to functionality to maintain its aesthetic. If it was willing to abandon all else in favor of raw power, it would hardly be better than the adventurers grinding mindlessly within it.
A secondary problem was the value of the manacrystal might be even more absurd. If manastone was valuable enough to be worth taking, then manacrystal, with its ten-thousandfold cost, might be valuable enough to cause serious problems.
What it really needed was a materials lab. Obviously, that was its entire dungeon to a point, but if it was looking to prevent thieves mining it for its manastone, it would like to be more proactive than incrementally proofing itself against each theft as they bypassed its defenses. It wanted to present an unassailable front.
It had many useless rooms in its lower warrens, and so it repurposed one of the collections of cubic chambers. It had just begun synthesizing materials and conceptualizing the tests it could run -- it had plenty of hammers and blades it could impact into walls, in addition to explosive and mana-based attacks -- when noise from its newly-discovered entrance distracted it. It had been two weeks. It was troubling to have to work on the scale of adventurers; it reminded it of being weak and desperate in its early years, when the minutes it took to spawn a new mob spelled the difference between a party making it to its core or not.
In its haste, it just messily doped its black glass walls with scattershot workings of iron and aluminum, in the hopes that would be enough to reinforce it. The ungainly transformation left spiderweb fractals across the surface, audibly crackling, and gave the dark material a cloudy sheen. Not an entirely unattractive look, but it would have preferred to have time to refine the material before deploying it. Or to run any materials tests on it.
The figures approaching its entrance cohered into vague lumps. Goblin Cave had gotten good enough at parsing the images to comprehend which way they were facing, how they turned their heads.
"This is it," a voice said, and then: "No, don't touch it!" Another member of the party had stepped closer, still quite distant from its mana lock.
"We'll set up under those trees," they said, pointing. Goblin Cave was very proud it could make out the gesture.
Also, those were trees? The white-and-orange-red blurs, or the brown-and-green ones? Or both? It had read about trees: forests were comprised of trees, and they were used to produce lumber, and they had leaves, which were of different shapes. They had things called boughs. Goblin Cave had imagined them more like crystal spurs, full of sockets where they grew out the wood. For all that it had a panoply of rock materials, it had never really been able to synthesize wood. Much too messy compared to rocks.
Goblin Cave added 'unlock more non-fungal vegetation to see what kind of trees were in the system list' to its already incredibly long to-do list. The question of if they were in the system list wasn't really in question: Darkwood Grove was still around and still forest-themed, and Goblin Cave couldn't imagine it doing that without having some trees to spawn.
In any case, the latest adventurers set up a more robust camp. There were more boxy, angular shapes unfurled against the backdrop of trees, and most of it was outside its field of view.
Later, they assembled near its entrance: "We're going to be running this as a dungeon evaluation, even though..." the voice trailed off. "We don't know. I know none of us believed the reports we heard, but looking at it, calling it a nascent dungeon might be the best category we can come up with. But we don't know. Anything could be in there, so don't get lazy. Stay within sight of your assigned trio at all times, and if you ever get separated even for a moment your first order is to retreat outside. You see anything strange, call it out. You have any innates that go off, tell everybody. Clear?"
There was a general motion and mumble of approval.
Goblin Cave felt like, at last, it had met a kindred spirit. Seeking knowledge, aware of the uncertainty, thinking of possibilities they couldn't forsee in the moment: this was, at least, thinking more in line with its own thoughts than anything it had heard before.
"First day, we're only doing light recon. If we end up with access to manastone reserves of the same purity as the samples, we'll be able to get mana supplies on site, but --" They looked over at the entrance. "It's hard to say. Those samples we saw don't look like they came from the entrance, so they could've been from further in. But if so, we'll be doing heavier exploration tomorrow with a much bigger mana budget, so if you need a day to build up charge for spells, you'll want to start now."
And there was the part with the manastone. What was it valuable for? From context it sounded like it empowered their spells? Or powered them to begin with? Something like that. It couldn't say it had ever noticed anything like that being used by the adventurers in its cave, and it would have noticed if they came in laden with manastones, but... clearly they wanted manastone for something. If nothing else, their encampment outside was encouraging: it might actually get to see what they were taking the manastones for.
With that, their group headed closer to its entrance. At the very lip of its mana lock: "This must be the artifact we received fragments of." Goblin Cave felt a surge of validation: of course it was fragmented. "The mithril threading... that alone would make this artifact-level. And here's another one. Or, a replacement?" Goblin Cave was guessing about 'mithril' there; the book that had included manastone had no entry that seemed to match mithril: a green-blue mineral that exhibited a characteristic scalloped surface in pure form, which effortlessly absorbed and conducted mana. But Goblin Cave couldn't think of anything else in the lock that was valuable and could be described as threading.
The person pressed their hand through, moving their face. An expression. Then they pulled their hand back and stepped through, body shuddering at the sudden immersion in Goblin Cave's mana flow. Goblin Cave's sense of them billowed into existence.
"This mana..." they said. "I don't know what this is, but this is no dungeon. Everyone, come in one at a time, giving yourself time to acclimate. When everyone's ready, we'll head deeper."