Perhaps you are visiting your granny, or out shopping at the supermarket, or having a sleepover at your friend’s house. Maybe you’re bored to tears of driving in the car and begging your parents to just stop at that sideshow attraction, just to get you out of the car. You enter the building, unexpecting the unexpected. Which is when it hits you -
You know your grandma has always been chubby… but has she always been hunchbacked?
You know that supermarket cashiers are known for their poor complexion… but can acne give you gray skin?
You were expecting the cheesy little doo-dads at this roadside attraction… but that head seems to be looking at you, no matter where you go…
This is the world where that which you fear becomes real…
and you are completely unprepared.
After that, take a look at your wiki. There is some more helpful info there.