Authors note: this is completely un-beta read as I was booted from the Tensura Discords without reason a while ago. My plot-bunny here is just a focus on slimes, with some inspiration/creative license from the OVA episode where Rimuru meets the slimes, starting at the very beginning, as the Tensura world despite its MC being a slime doesn't delve too deep into slimes… so let's shake that up.
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A Slime's Journey

In the beginning the world was at peace as the Creator Dragon Veldanava had designed it all in harmony.

Everyday the sun would shine it's glory down onto the world, and from that brought life, the forests and fields were alive and all the many races within them shared the world that gave them life.

With the warmth of life the races could grow plentiful crops and the scattering of races soon grew form small groups of families to villages.
And with this growth came children and their laughter of play as they played through the forests of fields of the new virginal world full of hope and light and joy for the future.

Such as the playing happening here in the small elven village of El-Ru as today was one of the special days of the year in the world as something all children loved was happening.

The slimes were making their way through the region on the great slime migration.

The slimes, a race so unlike any other, silent and mysterious in their ways and behaviors, the weakest of all the races and benign in all ways, they worked for nothing but to serve the great duty of keeping the world clean as where ever there was life there was also waste and there were no better cleaners of the world then the slimes as wherever they went was left better for it as they removed the detritus of Veldanava's creation.
As a benefit of their migrations, the slimes also liked to play, children loved the smooth round blue bodies of the slimes, to hold or play play bouncing games with them, where the slimes bounced the children would bounce.

It could go on all day, children laughing and playing with the slimes until their parents called them in at dark and by morning the slimes will have moved on again leaving behind happy children with the fond memories of a day of happy play with the mysterious slimes.

~SJ~

~SJ~

The unnamed slime of the multitude was amused by the playing of the juvenile solid that was bouncing on it's back and spitting air vibrations at it, using it's lower appendages to straddle the slime as the juvenile "rode" it and waved a stick in the air.
To interact with the solid races of creation was always so very amusing for the slimes, so alien were the solids, covering themselves in strange dead plant and animal matter and staying in their few places and often huddling around heat-combustion within strange constructions of dead forest.

The slime turned it's limited senses to study the juvenile riding it, through vibrations and magicules the slimes sensed the world around them and from that it could "see" the one riding it was of the long ear variety, not that the slime knew, or even cared, why there were distinctions and differences between the solids.

The slime could tell that the light was going away soon so the signal was passed amongst the multitude and the other slimes who all agreed and began herding the solid juveniles back to their settlement where their solid progenitors were waiting, and with the young deposited back the slimes once again continued their great never-ending migration over the world.
Slimes did not stop and did not tire, that it was nearly dark now was of no concern for slimes as they did not need the light, the way they sensed the world was beyond such needs.

One problem for the slimes though was that they were not fast as a collective. An individual slime could move very fast if needed but that was not their way, they did not need to be fast as the multitude flowed through the forests collecting and eating any debris and dead matter, not that they actually needed to as the magicules in the air were the true sustenance of the slimes, but the creator dragon made them for this reason, for this duty, and as they knew their place in the world it was what they did.

[DANGER!] an alert from a slime was broadcast through the great magicule link.

Slimes did not posses words or even a concept of language, they communicated in the most intimate ways possible, a mind-to-mind communication with their memories, their emotions and their souls.

One alert of [DANGER!] became dozens, then hundreds, then thousands, the shouting of emosions was overwhelming.

Then came the screams of [PAIN!] from the slimes on the outer edges of the multitude were crying out with their souls.

Confusion and panic set into the slimes, the great horde was tossed into chaos as they could feel all around them the life-energy of fellow slimes were being snuffed out.

[DANGER!] and [PAIN!] were screamed out and a life of a slime was snuffed out, it was deafening, the calm order of the slimes was crushed, the herd was broken as slimes were scattering, slimes colliding with others trying to get away, something the slimes had never before experienced in their time since creation; [TERROR!] .

The slime was seperated from its part of the collective, it and a few others had broken away in panic and were now separated so it could now sense the terror of what was cutting through them.

Solids!

There were solids carrying sticks with heat/combustion in one appendage and in the other appendage a long stick with a pointed rock they were stabbing into the slimes, while other solids were using a long stick to send a pointed stick flying through the air to pierce the slimes.

WHY!? Why would the solids do this!?

Across the macicule link the slimes all saw the same and they could all feel it, the memories, the emotions and souls shared, that every minute was a cry of [PAIN!] and then death of a slime, hundreds and hundreds and hundreds.

The slimes having never encountered anything like this, having no concept of an "attack", panicked and had been bouncing away in many directions to get away, scattering, many bouncing back in the direction they had come from.
Slimes emerged from the forest back into the cleared settlement they had left just hours earlier, but it was not the same as they had left it.

Now it was full of heat/combustion and there were all sorts of solids running around spitting out so much of their air-vibrations at such frequencies the slimes could feel their skins ripple from it.
There were many more of the solids with the sticks with rocks and combustion sticks than there were of the long-ear type.

[PAIN!] the slime felt it in its own flesh, they had been followed through the forest by the attacking solids and this one felt it was pierced through its side.

The attackers moved on to the rest of the slimes, stabbing, clubbing, cutting… killing, they were giving off air-vibrations at the same frequencies as the happy juveniles, these solids were having fun killing the slimes?!

[Why?! What did we do wrong?!] was a concept the came from it's soul at this realization.

It felt its fluids leaving its body with magicules leaking away, it rolled in pain, never in their life ever experiencing anything like the idea of pain.

It bumped into something, warm, but deceased, the slimes senses could tell it was a solid of the long-ears type, the pain overcame its limited thinking ability, it didn't want to die so the slime gave in to instinct to devour the dead thing while in its own way crying out with magicules to the dragon-creator [HELP!]

Trying to consume the dead solid while also leaking body fluid almost caused the slime to blank out in unconsciousness, but it then heard something new in the core of its soul from the World itself.

[ CONFIRMED; ACQUISITION OF UNIQUE SKILL "MIMIC ELF" ]
[ CONFIRMED; ACQUISITION OF EXTRA SKILL "RAPID HEALING" ]

[ USE SKILLS : YES/NO ? ]

The slime did not understand any of that but it did something new, it made a decision and selected [YES]

The pain that hit it next was unlike any other, far beyond the pain the wound to its side, as it felt its body warping and mutating in a strange and unfamiliar way as its very nature was being broken down and reformed it felt itself elongating, appendages were breaking away from the body and it felt it's consciousness moving from the core of its body to a new place, and like a snap, the process was over.

Where there had been a medium meter sized blue slime there was now a naked blue-haired elf on the ground on its hands and knees and with a blue colored wounded scar across its chest and belly..

And for the first time in the life of any slime, it opened eyes glowing an unnatural blue hue and saw with vision the world… and for its first sight ever it saw horror.
For the first time ever a slime could hear with ears, and the first sounds were terror.

It looked around itself and saw dozens upon dozens of deflated corpses of dead slimes, piles of blue glop littering the ground, it then looked up and saw the constructions of the long-ears… on fire, it saw the orange and red glows of fire for the first time and not just the sensation of heat/combustion that it knew before.

It saw the the ones that caused all this, the round-ear type of solids, they were chasing down the long-ear types, they were stabbing with their rock-sticks some of the larger adult long-ears and the other smaller adult long-ears were being herded and tied together.

It began to stand in an unfamiliar body, the instinct of the body-mimic telling it this was correct though not without unsteadily shaking in a new form.
Then it noticed that one of the round-ears noticed it and was coming its way shouting air-vibrations at it.

Sight. Vision. Sound. Fire. Pain. Emotions. The herd.

It could not think, too many sensations, terror and confusion was the only thing it was capable of now. It blindly turned and ran back into the the forest.

It ran.

And ran.
Heedlessly running through branches and brambles tearing and scratching the new mimicked skin, several times it fell in its run covering itself in mud of the ground before it scrambled back up to run more.

Away. To get away.

It ran until it reached a clearing where the moonlight was radiantly illuminating the field… it came to an abrupt stop, its eyes so new and innocent opened impossibly wide to view the scene before it... the thousands and thousands of piles dead blue glop that was once the great slime migration that was glittering in the moonlight.
It could not be, but with eyes it saw, with magicules it sent out its senses as far as it could feel… only to get back a so few responses of life of terrified slimes scattered around the forest hiding.
The emotions cascaded within the slime, its new vision swam with water as tears welled up in its new eyes and dripped down its pale cheeks as its body dropped to its knees and for the first time ever a slime made a noise, a broken guttural keening wail screamed out from the lips of the slime and split the forest.

~SJ~

~SJ~

Gray.
The whole world is gray.
It was dawn and it was wet and cold and gray.

The forest was full of rainy mist, and moving through the mist was a dazed naked elf that was not an elf as it was neither male or female as other than it's long blue hair it had no gender features and being vaguely adult in size.
It was wandering without thought as it had spent all night pouring out its grief and listening through the magicules of other slimes now scattered and lost cry out.

How only yesterday it was comfortable among the multitude of slimes, a family beyond family, souls to souls shared, now it was alone.

It had spent some time feeling out the other slimes, individuals and small groups had survived… but they were all scattered out so far it could take weeks, maybe months to herd themselves back together.
But could it really be a herd if it was only a few dozens left alive?
So lost in slime-thought it barely noticed it entered a clearing.
It was back at the long-ears village.

How? How did it wind its way back here again? It didn't want to be here, but it moved anyway.
Could it really have been just the day before that a juvenile from this village was playing on its back?

This place was now a charred ruin.

To the slime the smell of acrid smell of smoke and ruin was new, but it did not like it.

Glowing blue eyes swept over the devastation, the settlement that was so lively was simply gone, the thatch and wood structures consumed by fire leaving only the foundation stones.

On the ground it noticed several of the round-eared ones, no life energy emanating from them at all, it approached one of them.
The round-ear was so hairy, covered in skins of animals and it was giving off such an unpleasant air, the slime had no basis for "smell" to compare to but it did not like the "smell" coming from the body. In the round-ears hand was a short stick with a sharpened rock tied to it and that it was stained with vibrant blue and crimson.

The slime scowled its mimic-face at the solid, then noticed it's other hand was clutching it's middle and there was something shiny there sticking out.

The slime reached down to grab the shiny and ignored the wet-squelch of it being pulled from dead flesh.

It looked at the shiny in it's mimicked hand, the thing had a handle on one side and was long and sharp and shiny on the other side and by the magicules the shiny had in it the slime could tell it was made by the long-ears as the magicules felt the same, the slime compared it to the wood and sharp rock of the round ear was holding, they were similar in a way… but the slime cocked its head to the side and decided, the shiny was better than the stick with rock.

For a reason it did not understand it kneeled down on the ground next to the smelly dead body and slowly pushed the shiny into the dead meat in a test, it did this several times, stabbing the meat, it was very easy.

It decided it would keep the shiny.

The slime was about to move on, still confused not knowing what to do next when it felt it… faint magicules, concentrated magicules coming from life.

It turned it's gaze to a patch of ground not far away, hidden under grass was a wood door and below that a hollow in the ground and from that hollow in the ground were five pulses of life-energy.

It moved the grass away revealing the door, and like so many things "door" was a new concept to the slime, but it managed to open it to reveal to the gray morning gloom five of the long-eared juveniles who looked up with tear-streaked faces at an unknown naked elf lady and they rushed out of their hidy-hole to hug the unknown elf and cry and babble their air-vibrations at it.

It looked down the tops of the five juvenile solid heads that were clutching it and wondered what to do now…

~SJ~

~SJ

In case you have to have it spelled out for you, this takes place WAY before Rimuru, this is an idea of Tensura told from a complete outside perspective.
If anyone wants to take this idea and run with it you have my blessing, just give a bit of credit, ok?

edit history, I forgot how FFnt effs up formatting, hope it displays correctly now.