general
This is an overtale AU, where the main undertale cast lives on the surface and the humans (Frisk, Chara, etc.) live under Mt. Ebbot. Hence the name overswap.
The monsters won the war due to superior strategy, numbers, and overall intelligence. They did not win due to strength, the humans are WAYYY stronger than monsters, just like in canon undertale. But, this time around, the monsters were able to outsmart the humans.
After the war, six human mages and six monsters met to discuss and sign a treaty. The humans agreed to go underground, using Mt. Ebbot as the entryway/exit. (this means the underground can, theoretically, expand outwards as far as they need assuming they dont try and return to the surface). The barrier can only be broken with power equal to that used to make it from the humans and monsters that made it. MEANING that the humans need six human mages and six monster willing to break it. The big issue tho is that humans kinda hate monsters (other than the few in the ruins and snowdin— those guys have come to disagree with everyone else thanks to the cruel treatment of anything/anyone magical, which includes monsters and humans and… whatever else. Snowdin and the ruins are pretty much a refuge for any mages or monsters in the underground lol, there’s like three people there that can’t use magic lol) and they hate mages. The way the humans have thought to do this is to “collect” (capture) six monsters, then convince six of the most powerful mages to “team up with” (use) the monsters to break the barrier. And then, of course, they’re gonna wage war on humanity. They also are actively trying to eradicate mages from their society because “magic = evil” and they’re kinda just assuming that when the time comes, mages will just suddenly decide to come out and help. They’re so fucking stupid ohmygoshhh.
On the topic of souls— the red soul doesn’t exist, since i hc that its a symbol of the player’s control instead of a trait. The legend of the delta rune still exists, with the player (in the form of the vessel) being the angel. Ofc this au isnt a fangame just a bunch of art and a (wip) story so the player is just gonna be like… idk a self insert of something lol. Divine intervention.
I would also like to add- humans have had a lot of time to figure out how to get their technology to work underground so there’ll probably be a bunch of more sci-fi sort of tech stuff in this au if i decide to add it. As is- cyborgs and robots definitely exist. These came before the CORE was invented, and were what ultimately led to its invention. Since it’s in such a hot area, the humans need special gear to be there and do the work needed to maintain it. Actually setting it up required using robots (very heat resistant robots) to actually build the stuff.
Monsters live in a relatively normal and peaceful society. They do have issues in terms of the humans. There are two groups: the ones that want to try to free the humans, and the ones that think that it’s a bad idea. It’s pretty much the big issue. Those who want to free the humans state that it’s wrong to keep them trapped underground like that, for reasons they don’t even remember. Those against the idea think that releasing the humans will only lead to more monster/human conflict and the possibility of monsters ending up underground. Monsters barely won the war the first time.
Okokok so hear me out— all the natural water is toxic and filled to the brim with extremely dangerous fish and eels and stuff. Soooooo what do people drink? They rely on an amber liquid known as brosh. Brosh is a lot like super watery honey, but not as sweet and a little tangy. There are also a lot of alien-like foliage (which also extends to being in the water) and the grasses are more vibrantly colored, like a greenish-blue or orange. (think of slime rancher)
All the big/important cities are solarpunk. The smaller cities and towns and stuff are also solarpunk-ish but they’ve been slower to embrace it.
Humans are trapped underground thanks to the monsters so they have that going for them… yeah they kinda hate everybody now. Monsters are friends with everyone. Nobody knows what's going on with the ents and they’re kinda scared of them. Everyone hates mermaids. The Androids keep to themselves out in the wilderness so people see them as being pretty chill. And by that I mean that they all live either alone or in groups of twos and threes in cabins out away from the cities and civilisation in general.
ALSO when I said that all the monster cities are at the bases of mountains that also means that when there’s a chain the cities kind of clump up and from a distance look like one continuous city. It would be denser in the middle, thin out as you reach the edge, and then get denser as you get closer to the center of the next city. They also have small little communities up on the mountains, though there aren’t really that many that live up there. ALSOALSO their cities make circles around mountains and chains lol
When it rains, the rain itself isn’t toxic since the chemicals in the water that make it toxic don’t go with it when it evaporates, so people collect barrels of rainwater and sell it as a special drink. After it rains, all the water sinking into the soil causes poisonous chemicals to come up and out of the ground, so people have to wear masks when they go outside. There are monsters that are immune to the toxins in the water or that come up after the rain, but they’re a minority.
Humans live in a society that frowns upon magic yet needs it in order to break the barrier. This leads to them collecting monster souls (capturning and holding hostage any monster that falls and hoping they don't die before they get enough) and oppressing mages (humans that can use magic) while also trying to manipulate the more powerful ones into helping with the barrier project. The royal scientist, for example, has a “volunteer” mage that is actively being studied in order to gain a better understanding of magic and the barrier as a whole, tho the mage isn’t being treated very well but theoretically speaking, they could stop if they wanted (it would just mean that everything would go to shit for them). This is why papyrus and ashley have to stay hidden in snowdin and the ruins.
In snowdin and the ruins, the humans are more accepting of magic and monsters and are against the rest of the underground’s view of them only being good for breaking the barrier, and other than that being barbaric and vicious and overall horrible and deserving of death. The ruins and snowdin is a refuge for mages and monsters.
mechanics (as in video game shit): the inventories and battle system are canon. the battle system is a liiitttlleee different though, to make it a bit easier to actually, yknow, write and draw and stuff. When someone chooses to fight, if they dont have a weapon equipped (and if they can use magic) they dont get the weird boat thing for attacking, they just fling their attacks at their opponent. If someone has a weapon, then they get the weird boat UI thing. Cool? Cool. the world is made up of code and stuff, which is how the vessle became a corrupted weird creature that exists there. thhhatttsss preatty much it tho, okk byeee.
species
animals
ents/mermaids:
separate from monsters in that they are not made entirely of monsters. They did not participate in the war. Ents are peaceful and friends with monsters, they also avoid Mt Ebbot. The mermaids are chill with the monsters that live in the bodies of water with them.
androids
they were created by the monsters. Different from ghosts possesing robot bodies in that they are genuine artificial intelligence. Did not participate in the war.
extra stuff on the mermaids and underwater monsters
evil. They and all of the super dangerous water creatures and monsters are all super mean and will eat anything. Anything. Super sharp teeth and claws. They are not sirens, they cannot sing to save their lives and they do not give a shit who you are, just that you have a body that wont turn to dust that they can eat. Nothing can fulfill their hunger. The monsters that live in the water have turned more cruel and heartless though (cough underfell cough) in order to survive the harsh environment.