Psycho
Psycho is one of the most celebrated psychological thrillers of all time. When Marion is murdered in the shower by a shadow wearing a dress with their hair in abun, it appears that her killer is a woman—but later, it is revealed to be Norman Bates, who had assumed the persona of his deceased mother, who he murdered along with Marion and several others.
The Village
A cultish nineteenth-century village located deep in the forest lives in constant terror of the horrifying humanoid monsters that exist beyond the community's confines. Eventually, it is revealed that the real monster is modern society, and the village exists in the present day, after all. The Elders left civilization to cope with past traumas, raising their children to reject and fear the outside world.