A slime was little more than a thin membrane around a blob of viscous, high mana liquid. They had no brain. They had no anything. There was nothing to grant intelligence or to store memories. It would be an exaggeration to even say that they possessed instinct. They simply reacted to certain stimuli. They were attracted by sources of heat, and something brushing against their membrane would be engulfed by it and dissolved in the liquid within. Two mechanical rules that made up pretty much the entirety of their hunting strategy.
There was a slime named Blobby. It belonged to a dungeon named Erryn. Not that it knew its own name, or that of its creator. In the absence of any stimuli, it simply sat there and gently jiggled. The effects of the [Merciful] designation that granted its brother monster Zephyr the first vestiges of intelligence and will completely passed it by. It ignored all of the changes to the dungeon over the years. There were simply no stimuli caused by the changes that it needed to respond to.
At one point the mana turned bright. It permeated Blobby, changing it fundamentally. The jelly like substance shivered as it changed colour and started to glow. Blobby reached out a little in a few directions, acting as if it had been poked from all sides. Finding nothing it settled back down. And that was the extent of its reaction to having its whole species switched out underneath it.
Without the capacity to form memories, time was a useless concept for Blobby. Nevertheless, time passed. Erryn grew, purged the hex bomb corruption from the world, and even took possession of the System itself. She went on to begin restoration of the surface, spreading plant and animal life. When she had done all that she could, and time was needed for the surface to settle, she turned her attention back to the dungeon. There were a bunch of floors of slimes, followed by dozens more that were completely featureless with nothing but enchanted plates buried in the floor. Erryn could do far better than that now. She could make a proper labyrinth. Some floors grew fake brickwork, some natural looking caves. Some filled with water, some with lava. Some even had a faux sky, and gave an illusion of outdoors.
The other slimes on floor six popped into nothingness one by one, replaced by a colony of goblins. Blobby of course did not respond as its comrades vanished around it. Neither did it have any fear of disappearing itself, unable to recognise what was happening and having no capacity for fear in the first place. But it did not disappear. Rather, it was moved to a new room. It quivered in response to the suddenly shifted environment.
Erryn looked upon Blobby. Her first named monster. The first time the system had outright given up and <Error>'d at her. There was sentimentality there and Erryn wanted to keep it around as the boss of this floor, but it couldn't be left as it was. Ideally it would evolve into something unique, like Zephyr had. Erryn considered the main problem was that it was simply too stupid to be a boss monster. What modification could be made to a slime to rectify that? Erryn considered her own dungeon core. Bathed in a sea of mana, it was the seat of Erryn's soul and mind. What if one was modified and inserted into a slime?
Blobby's world changed. The limited senses of touch and heat perception that it previously had suddenly became so much more. Subtle vibrations of its membrane were picked up as sounds. The limited heat vision expanded, sensing a wider range of frequencies beyond the original infrared. Blobby saw colour. Blobby knew what colour was. It looked around in curiosity. Walls, floor, stairs. It was a dungeon monster. No, a dungeon boss. It knew what a dungeon boss was. Blobby sat stunned by its sudden sentience, trying to process all of the knowledge that was abruptly crammed into it.
"Oh, did it not work? That's a pity."
A voice. Words. But not sounds. This wasn't the sense of 'touch'; its membrane hadn't been stimulated. Then where had it come from? The words were just there inside its mind without apparently having come from anywhere. Blobby had a mind. Blobby knew that it was a slime, and slimes were not supposed to be clever. But here it was, deep in introspection of its own intelligence. So... Did what not work? The timing suggested a relationship with Blobby's own sudden sentience. There was another entity responsible, and it was looking for the results. Then sentience was not the goal, and Blobby was a failure? No, what had it done since then but sit still? It had 360 degree vision, so movement hadn't been required to look around. And sitting still was the behaviour of a regular slime. So it needed to show that it hadn't failed, and behave in a way that a regular slime would not.
Ideally it would talk back to whatever it was, but slimes were alas lacking in the mouth department. No, sounds might come out of mouths, but they didn't start there. Sounds were vibrations. Didn't humans have vocal cords, that vibrated as the starting point to produce words? Maybe if it jiggled its membrane in the right way, it could make words of its own? But that would take some experimentation and practise. It didn't have time right now. Besides, whatever it was speaking straight into its mind wasn't using sound, so if Blobby did there was no guarantee that this other entity would be able to hear it. It could casually use words like entity in a sentence! Even on the scale of dungeon monsters with brains, Blobby was a genius!
Deciding that it didn't have any more time to waste on thinking things through, Blobby simply started moving around, tracing the edges of a square. After doing that a few times, it switched to a triangle, then a pentagon. If the invisible telepathic entity didn't get the message from that, then there was no hope.
"What is it doing? Oh, I see, shapes! Awesome! Hey, Blobby, can you hear me? Squat for yes."
Blobby spread out, dropping its height. It occurred to it at this point that it was weird the way it had immediately trusted this voice. Why had it immediately wanted to show off its intelligence, when it may have been better to play it safe and spend more time figuring things out first? Even now, it knew that it could trust that voice, despite the complete lack of evidence. Oh great, it had been spun into existence full of blind faith. So much for being the worlds first intellectual slime. Then again, it was currently theorizing that this voice was responsible for its sentience in the first place. It would only be fair to have at least a modicum of respect for its creator. A creator which was fortunately not privy to any of Blobby's internal monologue.
"Yay, it was a success after all. Woohoo!"
Nope, forget the respect. Blobby was not going to respect anything that used 'woohoo' in a spoken sentence. Blobby had standards.
"Why the shapes though? I can't think of anything other than deliberately showing off. You started it right after I thought the experiment was a failure. No, right after I said to you that it was a failure. Aww, you heard me and wanted to prove to me that it worked? How sweet."
ok, maybe a little bit of respect. It thought Blobby was sweet.
"If you're that intelligent, we should be able to come up with a better method of communication than yes or no questions. Writing might be the simplest, if I can find you something to write with. To speak you'd have to vibrate your membrane or something at silly fast speeds. Do you know how to write?"
Of course Blobby knew how to write. It was a genius! It squatted down again.
"Great. Let me rustle you up something to write with."
The voice went away, and with it a feeling of attention that Blobby hadn't even noticed was there. It seemed to have some time to itself now, so out of curiosity it tried to find out what sort of noises it could make. Trying to vibrate its membrane was a non-starter. It couldn't vibrate the whole thing at once, because that would require changing its volume. It could vibrate a small patch, but the ripples just spread out and distorted the noises it was trying to make. It needed an isolated surface. It didn't really have any other surfaces though. No, wait, there was something else. Something floating inside its body that wasn't dissolving for some reason. It was perfectly spherical, and glowing yellow. Blobby was pretty sure that shouldn't be there. Slimes were made out of, well, slime. They didn't have random glowing floating lumps. And yet, as it looked at the sphere, Blobby realised that the sphere was simultaneously what it was looking with. It was a truly weird sensation. Okay, message received. Do not touch the sphere. The sphere is important.
It was also small, and floating completely isolated inside Blobby. It was, in other words, perfect. Ten minutes later Blobby felt the attention bearing down on it once again.
"Oh? You were trying to speak? Impressive. But until you get the hang of it, hopefully this will help."
A half metre black stick, a few centimetres in diameter and sharpened at one end, popped into existence. That was a neat trick, Blobby thought briefly, before deciding that it was actually rather mundane compared to making an intelligent slime. It wrapped itself around the stick, careful to avoid letting it through the membrane, and started training its writing skills on the wall. This was much easier than speaking, and it was able to produce something legible after a mere ten minutes of practise.
Blobby learnt that the voice was called Erryn, and it was the dungeon itself that was speaking. Blobby was indeed her creation and had been granted intelligence out of some sort of sentimentality. Blobby was pretty sure dungeons weren't supposed to be sentimental about their monsters, but neither were they supposed to be female. So the dungeon was a weirdo. That was fine; after all if it had been a regular dungeon Blobby wouldn't exist. Or at least, wouldn't have the intelligence to realise that it existed.
Erryn also pointed out that there was currently no intelligent life on the surface of the planet, making the existence of dungeon bosses rather redundant. At least for now. Apparently there was a plan in the works to do something about that. Erryn had made Blobby, so it saw no reason to doubt the claim. In any case, for now Blobby decided to leave the boss chamber for more interesting grounds, wandering over to a camp the floor's goblins had set up.
The goblins stared. Blobby stared back. The goblins continued to stare. Blobby stretched upwards, reshaping its membrane to form limbs, and doing a fairly good goblin impression. Aside from being yellow and glowy and translucent, but there wasn't much it could do about that. Its core floated in its chest. A chest that held two distinct bulges. If female was good enough for the creator, then it was damn well good enough for Blobby. The name Blobby wouldn't work with this shape though. A new one would be needed.
"Greetings" it said, in a beautiful and perfectly clipped voice.
The goblins stared harder, now with a distinct open mouthed component. The slime previously known as Blobby smiled. She had found her first new friends. And this was just one floor! How many more friends would she find in the rest of the dungeon?
Centuries later there were no tales of a fearsome boss of the sixth floor. Rather there were stories of the weird intelligent slime girl that let you pass without combat if you solved maths puzzles. She also had a tendency to start deeply philosophical debates with anyone who made the mistake of indulging her. There were also other rumours, spread among the younger and more adventurous delvers in whispers and with lots of blushing, about some of the really interesting tricks she could do with her slime.