Slender is a new indy game based around the mythos of 'slender man'. Many people are calling it the scariest video game ever. It's available for free at: http://slendergame.com/download.php so download it and find out for yourself. 

From Wikipedia:
"Slender begins with the player in a forest at night. The player (a young woman from the sound of her heavy breathing) is equipped with a flashlight with a battery which must be conserved. The player is able to jog for a short period of time before becoming tired, signaled by heavy breathing and slowing down to normal speed. There are 10 various landmarks scattered throughout the woods including vehicles, a building, a giant tree, rocks, brick walls, a stone pillar, a tunnel, and fuel tanks.[2] Dirt paths are present, but the player is able to travel in any direction and is only bound by fences which enclose the area.

Slender Man is an entity that follows the player through the forest. It resembles a tall, thin man with elongated arms dressed in a black suit with a featureless, pale white face. When the player catches a glimpse of Slender Man, a startling music sting is played, but only once, unless the Slender Man returns while searching for the player. As the Slender Man catches up to the player, the screen becomes distorted with static, and if the player looks at it directly at close range for an extended amount of time, the screen will fade to static and Slender Man's pale white, featureless face appears on the screen as the game ends. Slender Man can also turn the player around if he gets too close, also ending the game."

There are also quite a few reaction videos available on youtube for your viewing pleasure.