I really ought to read Dostoevsky's Notes from Underground. I know a little Dostoevsky. Frisk adores him, I suspect. But I'm speaking now of another Underground, the Undertale Underground.
If you've played the game and invested any thought into the admittedly barebones plot of the game, then certainly you've wasted at least a moment on wondering: just what was the "War between Humans and Monsters" really about? What came before? What precipitated the disaster? (And how on Earth did human magic work?—but that's way too big a topic for this post.)
One obvious thing must be said: every single piece of information we have about the War between Humans and Monsters, and how the Monsters came to be captive in the Underground and sealed behind a magical Barrier, comes from one of two sources: either from the Monsters' own records which Frisk reads while exploring the Underground, or from the nameless narrative voice of the game, who talks about the War in the opening title cards. (And yes, I know that I am often assumed to be the game's narrator. I choose to remain skeptical on that point—for whatever reason, I don't feel that I can commit to such assumptions, merely because they're broadly accepted.) We get no information about the War from an unquestionably human standpoint.
This raises a curious possibility: the notion that the Monsters do not correctly remember the War or the reasons for their captivity. They may recall the broad strokes correctly, but there's important details missing—at least, from the scanty records one sees while playing the game. They may have perceived the humans' attack as sudden and unprovoked, but there may have been some provocation—possibly something that they themselves regarded as trivial, but which struck fear into human opponents. They may have perceived the magic spell ("cast by seven of their greatest magicians") purely as imprisonment and punishment, but it's possible that the humans saw it as a reasonable compromise, or even as the Monsters' only salvation: seal them away from humanity, after which human beings could perhaps forget their fears about magical monsters, in a domain where the Monsters could build some kind of life away from humanity. The Monsters hate the Barrier, and yet it's possible that from the other side, the Barrier is a safeguard, keeping the Monsters invisible and forever hidden from human beings who might otherwise destroy them.
There's more I could say on this topic but it would all be far more speculative; I've tried to stick to reasonably safe inferences. This may seem pointless to most people but, well, it's important to me! I have felt some compulsion to explore this matter as thoroughly as possible, even though there's so little to go on. I am curious for example about the coincidence between seven mages (creating the Barrier) and seven human SOULs (needed to break the Barrier); that could be accidental similarity, or it could be Toby Fox making 7 into a motif—it's such an interesting number and makes for the powerful visual symbol of six SOULs in a hexagon with a seventh in the center. But the symmetry does feel obscurely appropriate in a magical sense, a sort of equivalent exchange of SOUL power.
I have also vaguely wondered about whether the magical working which created the Underground as a sort of bottle universe, sealed behind its Barrier, is something that would need renewal to remain powerful...which perhaps suggests some interesting questions about what happens to human beings who fall into the Underground. we know about a few of them, because they're specifically captured and their SOULs sealed up for a purpose, but does that really account for all human presence in the Underground? if a human being fell into the Underground and died without being found by a Monster, what would happen to their SOUL? But here I'm speculating wildly now.
~Chara of Pnictogen