All right I'm Crazy Dave and I'm gonna introduce you to 4 AU Undergrounds so you too can ask the Sans and Papyrus of that Underground questions!

1: UnderFell: Everyone minus FLOWEY and Frisk are evil.
Even Papyrus and Toriel. :(Not mine)

2: GenderSwap: All genders reversed. Made by me!

3: Blabe!Tale: Monsters don't get powers but instead they get blades! Made by me!

4:UnderSwap: Character traits swapped between opposite characters.(not mine)

First book and chapter of said book!!!