Fuck, I'm bad at drawing. Just think of this as the Slender Man as drawn by Rob Liefeld.
Take note: it's not Slenderman, it's the Slender Man. He's not Jewish. (Well, maybe he is. That would be a strange Bar Mitza - "Today, you are no longer a Slenderboy, you are a Slenderman!" Fuck, my mind goes to weird places.)
There's a fuckton written about this guy. Seriously, just enter his name in Google and you will find a ton of stories about him, most of them just plain old fiction. Some, however, are true. It's hard to find those nuggets of truth in the steaming pile of stories, though.
Here's what is generally accepted about him: he likes to abduct children. He has no face and doesn't speak, can turn his arms into tentacles sometimes (or make himself have four arms/tentacles or something). He's been called everything from a fairy to an alien to a tulpa (Google it, I don't want to explain), but pretty much nothing is known beyond the fact that he is one scary motherfucker.
I've never personally seen him and I count myself lucky because of that.
Oh yeah, and he sometimes uses human agents called "proxies" or "hallowed" or "hollowed" or whatever. Basically, he can make humans do whatever he wants. Why he does this nobody knows.