Chara gets into Frisk's head. Together, they erase the world. And then Frisk regrets it, misses their friends and the underground and the happiness they once had together, so they come back. They reset, they return, and decide it's nothing but True Pacifist from this point on, because the monsters deserve to be free. The only problem is, Chara is still in Frisk's head- and Sans is remembering a lot more than he should. - - - "there isn't any LOVE in your soul," Sans adds. "i don't know how you managed that, considering-" suddenly, his eye sockets go completely black- "you're definitely a murderer."