Gastale AU.
A city filled up with unknown poisoned gas so the people in there tend to be on higher buildings in order to stay away from it, they wear gasmask in order to protect themselves without it they tend to lose their minds and their souls got corrupted and turned them into killing machines. Even as I say this some monsters might have different affects on them.
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Gastale AU Setting
After frisk did the true pasafist ending and geting everyone to the surface things went well fora few years. Soon gases got leaked into the world and now everyone needs to wear a mask. Alphys has seen what can happen to people that don't. She has been trying to find a cure for it sadly none has been found. So now people jump from rooftop to rooftop where the air it a bit clearer. Even though that is true they are no more stary nights you can hardly seen the full moons when it happens. For the sun that has gone hotter and can always break though the sky.

The buildings can range from different sizes. Everyone tries to keep in the big city with bigger and taller buildings. Plus that is where most of the safe spaces are located. In the safe places you can take off the mask you wear. But you breathe any gas it will slowly drive you crazy.

Sometimes you will have to go to other places to get things for the city just beware.
Also this is a rough idea of my version of this so if you join things will most likely change a bit.
I still have a lot of character to do if you want to do them you can ask.
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Just ask to join!!