Not all Nightmares are bad (Nightmare Sans X reader)

(EDIT: STORY IS BEING REWRITTEN! PLEASE CONSIDER REREADING THE REWRITTEN CHAPTERS [chpts. 1-31] ON MY PG!) You are a young teenager girl. You have a happy normal life........kinda. You're best friends with the angel of monsters, Frisk. One day you follow her and sans to strange place on accident.You see lots of hers a...