It sticks to canon with this plot: Chara, being annoyed with watching their family d!e, tells Sans the only way to finally beat the human is to k!ll everyone first. Sans doesn't listen, and gets k!lled the next reset. Chara tells him again, and, seeing no other choice, he listens. He k!lls Papyrus, who, unlike normal Phantom Papyrus, is not a hallucination. Papyrus constantly begs his brother to stop. He won't listen, of course. Sans is able to k!ll everyone minus Mettaton, who stayed in box form. His final stats: 19 ATK, 19 HP, and 5 DEF.
