So its called Facadetale, and it’s like, everyone in the underground is sickeningly sweet and perfect. basically, the underground is/was an absolute sorrowful mess but Gaster created a machine that basically created a mask for the entire underground that even the inhabitants of the underground are fooled by. Sans is the only one who knows about it, and he’s the one who keeps the machine running. (it’s the machine behind the tarp in the lab) In the Pacifist run, you don’t REALLY find out about it for sure until you release the monsters through the seal and they step out of the facade, but in the Genocide run, you break the machine really early on in the game, which ultimately turns the whole (now saddened and enraged since the Facade has left them) underground against you. The one thing that originally hints to it, is in the beginning of the game when Flowey talks to you. He asks, “Are you one to believe everything you see?” and you have no other choice to answer with besides “yes” and he answers back to that by saying, “Then trust me when I say, THIS IS FOR YOUR OWN GOOD.” and then attacks you. Then, you know, Toriel shows up and the game plays out.