The lore

Once upon a time in 10XX some adult fall in a hole and found out the underground of deltarune.

they try to figure out how to leave but they failed, some monster see them and show how is going down here.

but the adult become crazy so they create weapon and soul extractor and use overused the reset. one day they found the glitch reset and when they press it the monster become crazy and can't really die so they killed the adult.