No, I'm not joking.
If you've ever played Amnesia, this game is ten times worse.
The goal: collect eight pages of a journal hidden around the map
The catch: Slender Man is hunting you in the middle of the woods at night, and every few seconds teleports to a location on the map. The more pages you have, the closer and closer he gets. If he sees you, you have to run.
So far, very few people have beaten this game.
It has insanely creepy atmosphere and sound effects, and is free to boot.
I highly recommend this game.