Author's Note: I hope you all enjoy the story I'm about to begin writing, I originally had the idea when I thought of what Rimuru would do if he decided "fuck it" and utilized his knowledge of the modern world to effectively conqueror his new one by force. Course it's a bit more complex than that, and the general idea of the whole story is basically what if Rimuru became a "villain" in most eyes, or technically more like a neutral character. Not to spoil it, but that's basically how it'll go. Has this idea been done before? I dunno. Has it been done this exact way before? I dunno. Regardless, just know I don't mean to copy anyone, and if my work ends up doing that, oh well, I'm sorry, but I've read only one Fanfic really (so far) and that's the one I've favorite on my profile (It's actually quite good for those who like Overlord and Youjo Senki).
Lastly, seeing how the Prologue was met with decent approval by a number of people, I've decided to make a Discord Server where people can discuss Tensura related things and give feedback on the series if they so choose. I'm still new to this website, and just as new to writing fanfictions, though I would consider myself a decent writer at least. Without further ado, below is the discord, and lets get started with Chapter 1!
Discord Link: UC2wdrnjNj

Fury of Tempest

Chapter One: Fall of the Federation

It all had happened a week earlier, the Kingdom of Falmuth ultimately decided to launch their attack about a week early, leaving Rimuru back with the kids while Tempest was attacked. By the time Rimuru left and fought Hinata, Tempest had already fallen for a while, the shields had been down, and all the main protectors, at least those who weren't outside of the barrier or hidden underground like Geld, had died.

And he was pissed.

Nevertheless, that wasn't yet. He quickly teleported into the sealed cave, his instinct giving him a feeling it was the best option, as he met up with Souei and the others hiding out in the cave, considering their next move.

"Master Rimuru! We were worried after we couldn't contact you, are you alright?!" Souei says slightly panicked, the first time Rimuru, or really anyone for that matter saw him break his more formal or serious stature, a clear indication something has gone terribly wrong.

"I'm fine, Souei, what's happened." Rimuru says, hopping off of Ranga and into human form as he gazes around, everyone seemingly avoiding eye contact.

"W-well you see, Sir…" Souei responds, drifting off as he himself couldn't make eye contact, looking at the walls and floor of the cave as he spoke

"Spit it out, Souei. Tell me what's happened."

"Tempest… It's gone, Sir… We-no I failed you Master. There was a barrier put up, and those inside were both weakened and couldn't escape. The Kingdom of Falmuth Invaded, and as far as we know… everyone inside Tempest is dead sir…" Souei says depressingly, himself barely capable of telling his Master of his failure, despite it not really being his fault. Regardless it had the expected effect, his master was clearly not happy, though his anger was not directed at him. His aura, normally collected and confined to the point where one would think him to be no more than a human, or at worst a weak monster, now effectively exploded out of him like a waterfall, a presence unlike they had seen since fighting the Orc Disaster. The main difference was, this aura was one filled with anger, not blind hunger which plagued the Demon Lord Geld.

Rimuru turned away, putting on a replication of the mask left to him by Shizu, looking back as he asked "Are their armies still there?"

"I believe so, and the barriers seem to also be down now. There may be some refugees, we're unsure. We were planning to launch a quick search and rescue mission to get them out." Souei responded, the others behind him seemingly realized what Souei, who in their own mourning, hadn't noticed yet…

"There's no need, Souei. I'll be back." As Rimuru spoke, his wings sprung out of his back as he took off, a small crack appearing on the side of his mask, allowing some of his anger-filled aura to leak out. Souei was left there speechless as he realized what the others already had.

Rimuru was going to face Falmuth.

Alone.

He was far above what was Rimuru City, now a Falmuth military base. He gazed down, seeing what his dead comrades buried in a mass grave where soldiers appeared to be covering their bodies in dirt. It was clear with inspection that their bodies were likely burned for a while before they were buried.

"Disgraceful scum."

As he gazed upon the disgrace of his friends and citizens as well as the city itself, he grew with anger, the mask yet again cracking as more of his aura leaked out, though he was still far enough above the ground that nobody had yet detected it.

"Notice: the most efficient way to eradicate all Falmuth forces is through the use of the Physics Magic: Megiddo." Rimuru could hear Great Sage's suggestion. And it wasn't a bad one either, however it had one major flaw:

The death was painless.

"Great Sage, optimize the most painful way to kill Farmuth's Army while still maintaining as much efficiency as possible." He thought for a moment, eventually deciding against his decision, "Actually, on second thought, would it be possible to hurt those who are more guilty than those who aren't? I'm sure some of these people didn't support this war, but that doesn't mean I'll spare them. Regardless, those who have had no will to cause harm to Tempestians deserve the only mercy I will grant them; a painless death."

"Analyzing..." Great Sage Calculated for a few seconds before continuing; "Created the Unique Skill: 'Maakheru' to fit your request. Those deemed 'innocent' in your eyes will receive a painless death, those who are deemed guilty will receive deaths perceived as painful and long, getting progressively worse based on how 'guilty' they are deemed."

"Perfect. Brilliant work Great Sage. Now let's begin." As he says this the crack expands, now moving over much of the upper right portion of the mask as he begins to activate his Unique Skill.

It begins with a ring of black flames surrounding the town swirling around before a pulse of magicules fills the air, launching the flames high into the sky, engulfing Falmuth's forces in a flaming tornado, or a tempest if you will (ba dum tss). As the flames swirled, so did the wind, as the Falmuth soldiers looked around and panicked.

"May the Gods' judgement be blind and merciful, for I shall not be."

"MAAKHERU!"

As he said that his mask cracked, his aura erupting as his left eye became visible, a shimmering red light emitting from it, the rage of Tempest had fallen upon Falmuth's army, and as quickly as it began, it would end. One by one each soldier of Falmuth would combust into black flames from the inside out. For some, it was a painless process, a simple thought and they were dead. For others, however, their deaths felt like minutes, hours, even days for some as they felt every fraction of a second of burning. Unlike Megiddo, which utilized water droplets to reflect beams of light, minimizing the magicule requirement down to practically nothing, Maakheru was a spell utilizing the full might of Rimuru's magicule stores and Great Sage's mind. In any other circumstance, this spell would be impractical or inefficient at best, but efficiency wasn't its purpose.

The wall of flames went high, and anyone within most of the Forest of Jura, and even in the nearby Kingdom of Blumund could sense, and some even see, the large spurge of magicules flowing out of Rimuru City. From the cave Rimuru's subordinates (what was left of them anyways), saw the flames, and immediately understood. Some had previously held doubts about whether or not he could take out their entire army, but those doubts were crushed alongside the army.

"Notice: All Falmuth Forces have been confirmed dead. Death total numbering over 10,000. Requirements have been met; The Harvest Festival to turn the individual Rimuru Tempest into a Demon Lord has begun. The Harvest Festival cannot be canceled once it has begun."

"Wait, Demon Lor-?!" Rimuru is cut off, struck by a wave of tiredness as he slowly falls to the ground, the scorched bodies of Falmuth's army surrounding him. "Ranga!" he calls out, quickly responded to by the lovable Tempest Star Wolf hopping out of his shadow. "Go inform the others, tell everyone to regroup here, Falmuth's army is gone."

"Yes Master, but what about you?" Ranga asks, head tilted as he was prepared to pounce away towards the cave.

"I'll be fine! Now go!" Rimuru calls back, Ranga quickly dashing off as Rimuru begins a demon summoning Ritual. "I offer all the bodies here as tribute." The bodies across the former City quickly collapse into clouds of dust as they swirl in the circle, swirling around like an explosion over the circle, revealing a powerful demon, donned with a suit along with his two subordinates.

"Ah what a familiar feeling, the Birth of a Demon Lord, and a powerful one at that. I will put my full effort forth to become your servant." the newly-summoned demon speaks, bowing before Rimuru laying in his slime form on the ground.

"Hey, uh yeah sure… just uh, make sure I'm protected… Not sure how long I'll be asleep for, but my subordinates should be arriving shortly, so don't attack them…" he quickly dozes off, the Demon placing him on a stand neatly as his slime body glows and shimmers like a birthing star.

"And so it begins, the dawn of a new era. I can only imagine how my master will shift the balance of the world. I truly do pity those who have made him their enemy." The Demon says, standing watch over Rimuru as he sends his subordinates to patrol and guard the city and nearby areas.

By the time Ranga reached the sealed cave, the Harvest Festival was already in full swing, and before he could explain what was happening, a voice echoed throughout the forest.

"Notice; The Harvest Festival to evolve the individual called Rimuru Tempest into a Demon Lord will now begin. Upon Completion, gifts will be distributed to all those connected to said individual."

"Demon Lord? Gifts? What's going on Ranga?" Souei asks, just about as surprised as everyone else at the words of the world speaking. Only a small number in the room recognized it as that, but none of them understood how, or more specifically, why Rimuru became a Demon Lord. Regardless they had to get to him.

"I don't know, Rimuru told me to gather everyone to regroup in the city, he is victorious!" As he says this however, Gabiru and his men fall to the ground unconscious, shortly followed by Souei and then Ranga, effectively leaving everyone but Vesta unconscious.

"Well this is quite the situation I've been put in… I would like to meet up with Rimuru, but I'm not going into that forest alone. For what it's worth, Rimuru's attack likely disrupted all of the monsters in this half of the forest. Oh well, that's not my problem, I'll just wait for these guys to wake up. Maybe I should check out those strange Elf drawings rimuru had hid in the cave…" Vesta muttered to himself, talking to literally nobody as he was left as the only conscious person in the cave.

Meanwhile underneath the City, where Geld and others were hiding they heard the words, in fact they heard it all, from Rimuru attack to the dying screams of the Army, and now the Words of the World. Recognizing the feeling, Geld could guess what was happening before it was spoken out for him, bringing back memories of the Orc Disaster; Rimuru was becoming a Demon Lord. Eventually the goblins and much of Geld's men fell asleep, Geld was questioning what the difference was between Rimuru's evolution and the Orc Disaster's, however on the other hand, Rimuru had named all of them, the former one to hold the name "Geld'' did not. Despite his will to remain awake, he eventually accepted it, having confidence that Rimuru had won, and that there was no longer a need to worry.

"Notice: Evolution from a Slime to a Demon Slime successful. Acquiring new skills; Infinite Regeneration, Universal Detect, Lord's Ambition, Enhanced Replication, and Universal Thread. Acquisition of Ultimate Skill: Lord of Wisdom, Raphael Successful. Evolution of Unique Skill: Gluttony into Ultimate Skill: Lord of Gluttony, Beelzebub successful."

"Notice: Request Received. Evolving and merging the Unique Skill: Maakheru along with a number of other Skills into Ultimate Skill: Lord of Caesar, Minerva. Acquisition Successful."

"Evolution Complete; Individual Rimuru Tempest has successfully evolved into a Demon Lord."