In the video here, you can see ol' slendy turn up right at the 2:30 mark. At which point the freaky little girl uses his powers to wreak havoc on the
stressed-looking sweaty, gross guy. Shouldn't have messed with the freaky little girl, guy.
Hey, turns out I like Skrillex videos.
Update: A reader has pointed out that the girl and the monster in the video have been generally held to be based on Annie and Nocturne from League of Legends, which adds another wrinkle to this whole video. Hey, that's kinda even cooler than if it was based on Slenderman. (Thanks, Ragweed!)