I have a theory a game theory
Viewing post in mystery ending wingdings translated!!!
the guy in the basement and you switch bodys and then he locks you in the basement so that he can trap you because he knew you were gonna go there one day from the very beginning and save him so that he kills everyone in the game and kills himself so that you will be trapped in his body for the rest of the game and that explains that the genocide ending is the final ending more will be explained from later endings that takes place within this theory
i have a theory in the video he uses undertale music alot and in the mystery ending it says stuff in wing dings who else speaks in wing dings a secret character you most likely know im talking about gaster but another thing that connects gaster is that in the files gaster is regarded as mystery man if it is gaster it would explain why they use the term mystery ending instead of secret and it refers to the character as him wich relates to how in undertale it never as been stated that his name was gaster but there was more like a cryptic code of secrets where you can peice them together to find out his name