(That's Apocalypse Falls Bill Cipher above.)

Stanford's portal malfunctioned causing an entire plague to consume Gravity Falls that mutated everyone into horrible creatures and the plague is slowly taking over the rest of globe.

Dipper- Dipper's bones have became weaker giving him wings, but he gets sick easily.

Mabel- When the plague started Mabel thought the only way to stop it from spreading was to kill so she became a Hunter and hunts down all the plagued humans or creatures.

Bill- Bill knew this was coming, he's been fused with the shapeshifter causing him to be even more dangerous than before.

Soos- The plague caused his personality to change making him viscious and slightly insane.

All of your characters have been mutated whether physically or mentally. They either have a personality change, or a physical change.

Can your characters survive the plague?

(Here's the Tumblr version of an explanation in case your still a bit confused. This AU was not my creation i don't know who made it but it wasn't me.)

Stan's dimensional portal malfunctioned causing This surge of energy and that caused destruction and distortion, messing up all of Gravity falls extremely, since it was the closest to the source, but still violently shaking and disturbing the rest of the world.

Because of this powerful surge of energy a odd plague occured that caused whoever was infected by it to be vulnerable to weird mutations. These mutations caused the infected to, morph half into creatures from things they've interacted with, such as DNA from a bird's feather in Dipper's case causing him to gain wings and a much weaker bone structure. He also gets sick a lot.

Soos- the mutations also warped their personalities, some moreso than others. Soos ended up turning vicious and slightly insane, and currently inhabits a graveyard which he guards with his life.

Bill Cipher is mutated as well, although in a different way. He's not stupid, he knew this was going to happen, and he used it to his advantage by making sure he was in contact with the Shapeshifter. Bill tends to prefer the form of a winged cat or fox, since while he can shapeshift, he is always glowing yellow, with one eye and his hat and bowtie. his shifting powers are extremely dangerous, as he can turn into, quite literally, anything.

Mabel- Mabel became a hunter, one of the people assigned to try and completely eradicate the plague so it doesn't spread. There were many ways to go about this, but after a while, Mabel ended up finding the easiest way was to kill.