Swapfell is an Underfell and Underswap AU combination, the fell-ified version of Underswap. This is also a community shared AU, so character designs and personalities may vary from each other. Instead of good natured monsters who don't actually want to hurt you, the monsters want to kill you. Instead of Temmie being evil, Temmie actually wants to help you escape this hell.
About the AU
Swapfell is similar to Underfell, except it has the characters of Underswap.
The characters in the AU behave like brats, they are mostly evil, they're usually yelling, and most, except for Papyrus and Temmie, are bullies. Papyrus is still slightly like the rest, but has hidden motives.
As Swapfell is the Fell-ified version of Underswap, there are several changes with the characters. They also commonly wear purple, since in Underfell, most character designs include the color red. The Underswap characters' personalities are the opposite.