(Cue: Undertale - Once upon a time, by Toby Fox)
The world of Undertale. The World is populated by the Monsters and Humans alike. There is peace, Peace between the two races, which seems to never have any reason to end.
HAD no reason to end.
A Human group one day decided to provoke the monster society, even though they had little to no reason to do so.
The monsters of course, gave the humans what they deserved. If you provoke someone, don't complain when they hit you back.
This the monsters did. They lightly wounded them, but mostly just made them run away. The Human group fled back to Human territory and reported a false story;
They said, that they had been simply hunting near monster territory's border and multiple monsters ambushed them for no reason.
The Humans in charge doubted this, as that group was known to be anti-monster and rather racist. They might be trying to get the two races to battle each other, increasing the chance of monsters getting killed.
But the suspicion and distrust remained nonetheless.
The people living nearer to monster territory started to go closer and closer to the monsters, invading the neutral zone with curious and suspicious people.
This of course, aroused the attention of the monsters. They could hardly ask the human government to call their people back, without strengthening the feeling that they might be hiding something.
Instead, they started setting up a highly increased amount of guards along their own border. Just in case the curious Humans entered their territory without permission, and endangered their public security.
You see, humans are naturally far more violent, immoral and unfriendly than monsters, stealing and murder for example, are a rarity beneath monsters. Letting in lots of Humans, most certainly would take a big toll on the public atmosphere, the long-built up general, nearly conditionless trust within the Monster territory would come crumbling down.
But setting up the guards had the same effect that the other countermeasure would've had.
The Humans felt like the monsters were hiding something, and spread rumors about it. The Human government heard about the monsters seemingly plotting something.
So they sent messengers, which had the mission to ask the monsters if what they were doing concerned the Humans too, for they were growing nervous. The messengers however, were stopped at the border and sent back, as this probably could encourage the curious ones to enter regardless of the guards, they could take the messengers as an excuse to enter, (like, "if they're allowed, we are too") for example.
The messengers reported of their repulsion.
A denied request of diplomacy. The unwillingness to resolve recent tensions peacefully. The conformation that something was up. As the tensions grew, the anti-monster group saw, and took their chance.
They began stirring up the humans by spreading rumors and false news, increasing the pressure on the government by a multitude. A time of political unrest began, comparable to tectonic plates, constantly charging up with potential energy, just waiting to finally discharge it in a massive earthquake.
This earthquake was soon to come.
The final nudge that released the earthquake, the spark that lit the fire, the last droplet in the full barrel, started brewing as one night multiple men, not even part of the anti-monster group, met together in a bar.
They started arguing together and drinking, talking about what the monsters might be up to. Their discussion became more and more heated as the alcohol levels rose, becoming offensive, accusing, aggressive and angry. It didn't take long for two of them to get into a physical argument, ending with one of the two knocked out. The men's "leader", saw this through the fog of alcohol, being a bit less drunk than the others, stopped everyone. He had an idea: How about telling those monsters, what humans thought of them? If they wouldn't understand, a few clubs and swords might show them...
They gathered the everyone in the bar, even those, which weren't part of their group, and wandered to the monster territory's border. There, the small mob of drunk men called the guards of the monsters names, provoking them and becoming more and more aggressive. It was in the darkness of night, so it was far harder for the monster guards to understand what was happening. The Humans became physically violent not long after.
The guard-monsters fought back in self-defense, but the provokers kept on going. This developed into a minor border conflict. It ended with the humans retreating the next morning, returning bruised and wounded. They sought out for revenge, which they brought over the course of the next day. The minor border conflict grew to a medium one.
The Humans which had attacked first, called for others to join in on the fight, to get revenge for their humiliation, to defend their territory and honor. The Monsters of course, didn't know this, it looked to them as if the Humans started an assault on their territory. The problem however, was that neither side really knew what was going on.
Who is fighting who? Were the humans ordered to fight us? Where are they going to attack next?
So this, like any conflict, began to escalate further and further. Humans from other border areas started to join in on the supposed defense effort, and the guard monsters had to call in reinforcements ...
Soon, this began to escalate into real, full-on-out WARFARE.
Precisely, into the Great Human-Monster War.
At the beginning, the justification was the distrust and suspicion of the other side planning to take over the other's territory. During the war, and at the end, they both fought no more than an endless cycle of revenge and counter attacks. One attacks the other, which they hit back a little bit harder, which the other has to then take revenge for.
This is the pure, unfiltered concept of war.
The longer it goes, the harder you have to strike back. This type of conflict...
It was unwinnable from the start. The monster civilization, less offensive but in far greater numbers and capable of magic, being slaughtered in dozens by mere single humans which all have a base level of DETERMINATION.
"The power, that lets you clench your teeth together, get right back up again, and KEEP ON GOING!!!"
A power, which monsters do not possess, in fact, do not have the capability to possess. Determination requires physical matter to contain in, in larger quantities at least. Humans have such substance. Monsters, mostly made up of magic, don't. To this comes that there are humans, which have a "traited"-soul, they are capable of magic. The magic they can preform, depends on their trait.
For example, a human with the "kindness" trait, (the green one) often has the capability to generate transparent, green glowing magic energy-shields to protect themselves or others, and most can heal others, too.
The true limit to a traited Humans' magic mostly lies within what they can imagine their magic to do, yet, the more "traditional" applications require less energy than coming up with some new form of use or attack.
However, since most humans aren't capable of magic, the main advantage the monsters had (next to larger quantities), was basically negated by the traited humans, commonly referred to as "Wizards" or "Magicians". While human magic has a far smaller range than monster magic, it still gave a boost to their forces.
Maybe it was the magicians, maybe the power of Determination, maybe the docile, non-aggressive culture of the monsters, maybe the Humans attacking first, or even all together, but the monsters lost the war.
They already were sustaining a high death count as the final blow was dealt when their leader, King Vailor, was assassinated.
His son, prince Asgore, only a young man, was declared King soon after (In the war, they had no time to hold a coronation, so he was declared king). In a valiant effort, he and a few of his friends entered the human territory alone and sought out the Human leaders, and declared the unconditional surrender of monsterkind.
The Humans accepted. This war had gone on too long, costed too many lives and resources...
The Humans didn't know what to do with the civilization at their hand. Brutally slaughtering them off, just didn't make much sense, they wouldn't gain anything out of it, either. And most importantly, the Humans had at least, a little bit of mercy.
Imprisoning them all was impossible, as they one day might rise from within the Human territory and destroy the humans from inside out, as there simply are too many of them. They'd have to build dozens of very large prisons, just to get them all beneath a barely secure roof. Building these prisons, the monsters could of course do, but they could add weaknesses to break out eventually...
Exiling them... where, really? They didn't live within, or extremely close to Human territory, so they couldn't just "go out", and besides, they might return with vengeance too.
Simply put, no one knew what to do with a very large amount of war prisoners, and them all being capable of magic, made every single civilian a potential threat, too.
Nothing happened for two entire weeks, the monsters tensely awaiting the Humans' judgement, patching up their wounded warriors, the Humans doing the same and trying to think of said judgement.
But then, a wise, experienced old man stepped up beneath the Humans, proposing a radically new solution:
There is a gigantic cave system beneath a far, far away long inactive volcano which is named Mount Ebott.
How about Exiling the monsters there? They'd have to march there on their own, human guards only ensuring that they arrive there and go inside the mountain. The cave system is deep enough underground, so that there are no other entrances than Mt. Ebott.
Then, six of the most powerful traited humans, one of each trait (except Determination, for that one is very, very rare) would work together, creating a spell that made a one-way seal, a barrier, to the entire cave system, so that no one could get out.
As Human souls are absurdly more powerful than monster ones, requiring roughly every monster's soul that the war left untouched, just to reach the power level of one human one, the barrier would be impossible to cross or breach for the monsters.
Because six Human souls created the barrier, it would need six Human souls to reach it's power level, and seven to overcome, to surpass it, to be able to destroy the barrier.
This plan saw broad acceptance, and the government let it be executed on the same day. King Asgore, having surrendered conditionlessly, had no choice, but to bow his head and accept the offer to either be executed, along with his people, or move with his people to Mt. Ebott, to forever stay underground, never seeing beautiful day- or starlight, precious trees, blue oceans, green grass fields... the SURFACE ever again.