Septictale is based off of Jacksepticeye's playthrough of Undertale. In this AU, Frisk's name is Sam and looks similar to Jack. Sam has the power to reset timelines and all that stuff, etc.
Besides the fact that many of the characters will have some color changes, Septictale is also corrupted. There has been a glitch that has spread throughout the timelines that caused the characters' codes to become corrupted, They call it Alter. When Sam first meets Flowey at the beginning of the game, he sees that Sam may be able to fix the corrupted timeline. He decides to accompany Sam in their journey to stop the glitch from taking hold of everything before it's too late.
Many of the main characters are corrupt because of the glitch, and their personalities are hostile. The hardest, most dangerous enemy is AlterGaster, which the glitch has brought back from the void. After Gaster is defeated, he is able to help them work for a cure to the glitch.
Eventually, Gaster is able to turn Flowey back into Asriel, and they all work for a cure in Alphys' secret laboratory. They manage to make a way to safely reset the timeline, mashing all of the uncorrupted timelines together.
The final and most challenging boss in the game is Anti, who is the Chara of Septictale. Anti was the one who created the glitch and caused chaos across the timelines. Sam decides to break the reset button so that the timeline cannot be reset and everyone can live in harmony again. Anti is reduced to a little septic eye that is kept in the secret lab.
I am currently working on getting drawings out for this AU. I am also working on an RPG for this AU. It will be a while until it's finished, but I loved creating this AU.