The story picks up right after Undertale's True Pacifist ending. Frisk escapes with their friends from the underground. Monsters finally roam free on the surface. Near the barrier is an island where they've decided to settle. Things were going great, everybody lived in peace. This is what every monster has been dreaming of when they were trapped underground. The Sun shined brightly onto the island. The monsters thought it looked beautiful, it's something they've never seen before. Every morning, they admired the sunlight as often as they could.
As the days pass, nobody even seemed to notice until it was too late. The sky was getting darker and darker. And then an enormous maw forms right at the heavens, this terrifies everyone. It was loud and growling, nobody knew what was happening. It's the Gnaw. Hungry, angry, and snarling. The Gnaw growls louder and louder, growing impatient at the confused monsters. It has waited long enough, it decides to punish them.
A plague is cast upon the island. The monsters stare in horror as some of their friends and families drop to the ground and beg for help as they slowly mutate into horribly disfigured scalies. Frisk found this to be unacceptable and went to civilise with the Gnaw. Attempting to reason with the Gnaw, Frisk pleads with it to reverse the effects of the plague and just become friends with everybody. The Gnaw ignores the pleas and nonchalantly turns Frisk into a scalie. Without even stopping to react, a mermified Frisk immediately jumps into the ocean, never to be seen again.
Frisk's friends are shocked at what had just happened. Everyone felt confused and frustrated. The monsters started looking for solutions. Toriel believes that the Gnaw wants sacrifices, but they didn't know what to give away. Alphys went to study the scalified monsters in hopes of restoring them back to their original form. Asgore was overcome by anger at what the Gnaw did to his people, he wanted justice for them and declares war. He organised his best soldiers, including Undyne, to attack with their magic powers. Asgore and his army gathered underneath the Gnaw and started throwing spears, fire attacks, and such in an attempt to vanquish it. This proves to be futile as it only served to further anger the beast.
As a punishment, the Gnaw scalifies all of the soldiers plus some other innocent monsters. Undyne and Alphys undergo the scalification process, but they remain unaltered since they're already scalies. Asgore almost turns into a scalie, but fortunately, due to Alphys's prior research on the scalies, she was able to significantly delay the process, although not completely halt it. Still, Asgore knows the inevitable. Still not satisfied, the Gnaw drops a few nukes on the island, destroying most homes and leaving only some living survivors. The monsters were devastated. Toriel, after witnessing the attack's failure, decides on a more peaceful approach.
Toriel runs up to her kitchen and started cooking up her best recipe, butterscotch-cinnamon pie. Toriel offers the pie and the Gnaw immediately beams it up to its maw. Everybody watches in delight as the pies travels up the beam and gets devoured. The Gnaw is so pleased that it rewards the monsters immensely, so it brings Flowey to the island. Flowey is perplexed as he looks around, seeing everyone staring at him. Then the Gnaw demonstrates its power by returning Flowey back into his original form, as Asriel. The monsters rejoice with renewed hope, they saw a miracle happen right in front of them. Asriel is tremendously ecstatic about retaining his real form, he no longer has to be a soulless flower anymore.
It has been realised that what the Gnaw wants is food, so Toriel cooks up all kinds of pies to sate its hunger. Pie after pie, the Gnaw gets spoiled, eventually losing interest in pies and started to develop different cravings. Toriel notices that the Gnaw has been gradually getting less satisfied, and so she started needing to cook various other kinds of recipes. She knew she couldn't do this anymore, even with the help of the remaining monsters, as the Gnaw gets progressively even more gluttonous and picky. The island runs low on resources, limiting her options.
Out of nowhere, a group of three humans suddenly pop into the island. And this is where we, the Don't Starve Together community, get involved to help the monsters!
We've developed this Undertale mod for Gorge to allow everyone to participate in this magnificent AU:
https://steamcommunity.com/sharedfiles/filedetails/?id=1426198349
How does this story end? It's all up to you!