Anything, from fan comics like "Dogs of Future Past" and "UnderRift", Fanfics like "A Year Every Minute", and even every single mention of this Character that claims to know his origin, WD Gaster is character with no dialog, that only appears once in every 500 or so playthroughs of this game. He's a mysterious character with 101 different interpretations, which let to the absolutely fabulous and fantastic EchoGaster Project, which you should totally watch right Here . But regardless, Gaster might be one of the biggest characters in the fandom.... despite being a complete mystery. Fascinating, Huh?
What are your favorite Undertale AUs? Any popular ones I missed? Comment on it!