It was something they came up with on some forums one day when they were holding a contest to fake "paranormal" photographs. The myth behind "slenderman" is a little misleading. It is a very creepy game because he teleports around and always teleports to a position out of sight and you look at him which starts the "going insane" part. He is not supposed to kill you in this, the idea is that he drives you insane and the level of static is an indication of how "insane" you are.
Some people here say Slenderman kills you, but when Slenderman shows up, if you look at him, your screen starts going fuzzy at a faster rate depending on how close he is to you and you eventually lose if your screen reaches full static. The game doesn't really have anything inappropriate about it.
I would not recommend it for children who are easily emotionally moved by games. This is one of the scariest games I've played. Oh and one last thing Slender man's minions are pretty freaky looking but not over the top freaky. He comes out of know where (as do his minions) and it is just so scary when that happens and another thing to note is that this game is very scary and that is why I put the age rating so high because it scared the sh** out of me at times! The last thing I well leave with is well the story is actually really good but you kind of have to paste it together off of notes and simmingly random items you pick up along the way. But never underestimate Slender man because he is one scary dude. And believe me it gets annoying when they get you because you have to start the level over again which is one reason why this game is repetitive. It can also be very frustrating because you have no way to fight any of Slendys zombie like minions. I think that people are (especially critics) are very harsh on this game because it is repetitive, which it is. But it is pretty pointless to some degree. Let me just say this is a great game, in fact one of my favorites.