Goopytale, or Goopy SOUL AU, is an Undertale AU created by SunnyCider, but was given to Arts Kislali after SunnyCider no longer wanted anything to do with the AU. It follows Frisk, now 17, who has fused with Gaster, causing their universe to collapse and monsters being released from the void.
Frisk first fell into the Underground and encountered WD Gaster in the Void by entering the gray door. They left, but ever since, they've been able to communicate with Gaster and see him. They become friends, and the reason why they can see each other (according to Gaster) is because of their "compatible SOULs", which makes Frisk a possible vessel for Gaster's SOUL which has been "stuck in the void outside of time and space". Frisk begins work with the aid of Gaster over the course of 5 years, and returns him to the real world after using the DT Extractor and fusing his SOUL with theirs. It works, but the event shatters their universe and opens several inter-dimensional rifts from the void, releasing Void Creatures- goopy, black creatures bent on wreaking havoc across the Multiverse. Frisk now has to travel through the Multiverse, gathering friends along the way, in order to stop the monsters from causing mass destruction everywhere.
Frisk is the main character of Goopytale and shares a body with Gaster. They are 17 in this AU and 5'11. They can use Gaster's magic, which is generating and using the "goop" that Gaster and the Void Monsters are made of to create virtually anything. Frisk is still is unable to properly control the power, so they must draw the magic from their part of their SOUL. They can't make very complicated things yet, so they usually use bones, Undyne's spears, and Chara's knife. They can be serious and scientific, but are sometimes making science jokes and generally being naive.
Gaster is the other main character in this AU, and, when, appearing to talk, usually forms as a mask on the side of Frisk's head, although he can presumably appear as his full self. He acts as an advice-giver and narrator for Frisk, similar to Chara in other AUs. Gaster can be happy to help, but can also be stern and isn't too much of a fan of puns. He is not related to Sans or Papyrus, and he was only Sans' coworker when he was still a Royal Scientist.