After the events of Undertale: Disbelief Papyrus, the human spares papyrus at phase 4 to resurrect the monsters from the dead, and break the barrier, after a few weeks or months, the human slaughtering any monsterkind in the surface, and so Sans himself, but he died.

And now Papyrus was now the one who watch his friends die to the population of the humans.

The human encountered Papyrus in the forest with river, only to fight or death.