Buttercups (Undertale AU)

This takes place long after the pacifist route, however, because monsters fought for their rights for three years on the surface.

Once they gain their rights, they return to the underground and pack for the official move to the surface. Flowey, however, is suspicious of Frisk, thinking that they spent far to long without resetting, and believing that they have a malicious plan. . . and he's not wrong.

Flowey comes up with a plan - he would poison the king, and use his sickness as a decoy while he causes chaos, knowing that Frisk would stick around if things got interesting, which would give him enough time to investigate Frisks plan and stop it.

With one of his petals, Flowey poisons the king, and begins to devise just how to pull off the chaos he's planning. Finally, he finds how he can cause the most chaos: he would explode the core.

With knowledge from past timelines, he tampers with the core, causing it to begin to destablize. This goes unnoticed by Alphys, who is packing, trying to figure out whats wrong with the king, and watching anime.

Eventually, small explosions begin to take place, and Alphys takes notice and enlists help in stopping it. This help includes Sans, Papyrus, and a mysterious third figure. They try to fix the issue, but its far to late, and the core explodes.

However, Sans, Papyrus, and Alphys manage to escape, but not unaffected. The explosion mutates nearly everyone in the blast radius, which is the underground and some land around it. Those who are not mutated die.

Deaths include: Asgore

Undyne

Mettaton

All the Temmies

The tiny froggit.

Mad dummy

Nabstablook

The mysterious third member

Bird that carries you over a disproportionally small gap.

The ThunderSnails.

Every Whimsun/Whimsalot

And finally...

the Annoying Dog, who is strangely replaced by the Annoying Cat.

Mutations consist of:

Sans(tentacles, large wings with almost no feathers)

Papyrus(larger eye sockets, broken off leg, a laugh that could probably kill a dog.)

Toriel(Eyelids burned off)

Frisk(Shark - like teeth)

Flowey(Black petals)

Chara(physical form, long hair that acts like Mr. Braves from My Hero Academia)

Alphys(Dosen't matter, she kills herself pretty early on.)

Loox(More eyes. a lot more)

The AU focuses on the lives of the monsters after the explosion, and, a little later on, their reaction to another fallen human.

have time good