Of course, Slender Man is scary when you're alone in the woods with only a flashlight. But when you're armed with a shotgun with infinite ammo, on the other hand, you can take care some dozen Slender Man clones in minutes.
Creepy FPS is a very simple game at all levels: the graphics and gameplay are basic, and you don't even need to install it. In fact, when you die, the game closes automatically. Still, it could be interesting if you really like Doom. Or not.