You fell into Mount Ebott, which was said to have monsters trapped in there, but...All you saw was dust....Monster Dust...When you eventually got to Snowdin, you saw...Sans, or to be more specific, Killer Sans, walking around Snowdin. When he notices you, he freezes in place, since, well, you are a human after all.

"Wha...Who are you...?"

Killer Sans would say in his laid-back, yet cautious voice. He is currently in stage 1, so he won't attack you, which means it's safe to talk to him. Just...Try not to get him to different stages...Or else you may die...You better be careful with your words or what you do...