Underswap (Underswap LC) is an AU where all of the characters' roles are swapped, as well as their personalities, though some quirks tend to stick in places, or are modified. Toriel is the timid queen of the Underground while Asgore is the fair caretaker of the Ruins. Sans is the excitable, confident, extremely optimistic human hunter, while Papyrus is the laid back, protective bro following Chara (who is swapped with Frisk). Other swaps in this AU are, Napstablook and Mettaton, Grillby and Muffet, Burgerpants and Nice Cream Rabbit, and Flowey and Temmie.
The difference is that Temmie takes two forms, swapping Asriel who is also Flowey. Her original with the name Leirsa and her temmie doll, (her former, lower self) who is aggressive and hostile.
This is an alteration to Underswap Team Switched which has Monster Kid as Asriel Dreemurr. Only this swap takes affect. There are no other alterations. Monster Kid can be swap with anyone else.
The story goes in the Comic: Underswap: Child of the Queen