Slenderman, that terrifying faceless ghoul in a suit, has been around for a good long while before that stripped-down, scary video game Slender hopped into the public eye. He's a long-standing meme going back years, and he's turned up everywhere from the YouTube Series Marble Hornets to... well, to a Skrillex video from last year.
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 guy.
Hey, turns out I like Skrillex videos.