So basically some of you may know of the game Slender, that being said I suppose you already know where i'm going with this...
Heres the pitch...
2 Sides (Standard idea) - The Hunters (Group of many whose loved ones have been taken away by them by the Slender Man)
Whether the Hunters are classed based or generically the same is up for debate...
Their Objective is to kill the Slender Man in the name of revenge.
Anyone who looks at the Slender Man for more than 5 seconds will experience effects such as blurry screens, breathing increases, accuracy decreases etc.
Same ideal setting as the original Slender game (In a dark forest, maybe urban ruins idn)
Slender Man - Has a health buff (amount of which is up for discussion)
Sort of like the Hidden... Except for minor changes to the abilities of the Slender Man.
The Slender Man can teleport to where he is looking (Use of Abilities has a recharge penalty)
He may also Teleport others to a random location to split up the group...
His primary attacks are not so much his hands but his tentacles that form from his back...
Once Slender Man's prey is low on health he can consume him by constricting him (Think L4D Smoker)
Slender Man is not agile, he can only sneak and slow run.
When he is not running his movement is completely silent.
Maybe for some comical relief add an ability to make a sharp alert sound like in the original Game Slender just to scare people.
Slender Man will know if someone is looking at him if they were looking at him for more than 5 seconds, there would be an icon on the hud to show it and the target looking at him would brighten up.
These are just some of my ideas whether you guys like them or not I wont be sure but... This is what I came up within 10 minutes of thinking of the Slender Mod for Gmod.