Recently, two twelve year old girls in Wisconsin stabbed their "friend" nineteen times and said they did it for "Slender Man". In Cincinnati, a mother says her daughter attacked her due to the influence of the infamous character. 
Many people have made claims of seeing the Long-armed One, and even claim he's been seen throughout history. Here we will explore the various claims and sightings of Slendy and see if we can get down to what or who Slender Man actually is.
History Of Fear Dubh
It is said that there have been sightings of Slender Man throughout history and one name attached to this fear-inspiring creature is Fear Dubh, among others.
It has been said he was seen in World World Two gathering victims in his arms as bombs dropped in Europe. It is said he had even been seen in the Vietnam War during firebombing of those fields in Southeast Asia. There are claims supposedly made by survivors of 9/11 that he was seen grabbing people from the collapsing buildings. In fact, stories of this long-limb menace go back to the 1400s, during the time of Vlad Dracul III, in which whomever Dracul did not kill, the Slender Man did.
Of course, it would be difficult to prove any of these sightings. It's very possible, though, that this gruesome humanoid is part of humanity's collective unconscious.
Yet More Claims
One of the long-lasting claims is that the Long Lanky One was seen in Sterling City, California back in the 1980s right around the time a group of children went missing and a library caught on fire. Some people have connected these events and insist the children were taken away by the infamous lanky man.
More recently the Lansing State Journal reported that a man in Mason, Michigan said his XBox and TV started changing channels and volume settings seemingly on their own; he told police that when he looked out his window he saw a tall thin man with long limbs walk away. 
There is even video proof that Fear Dubh was in Russia climbing a tall building and supposedly people have mysteriously disappeared from the area during the time he was seen there.
So What's The Truth?
The truth is that Slender Man is the creation of a man named Eric Knudsen, who goes by the user name Victor Surge. The website Something Awful had a forum thread in which users were invited to create pictures of the paranormal using photoshop. Eric Knudsen created the image of a faceless man with abnormally long limbs alongside a little girl. He added some back-story and various claims about incidents involving the creature to spice it up.
From there many stories were created, role playing games played and more images concocted. Variously, he is said to be able to grow tentacles out of his back and shoulders, to control the minds of children, to run at high rates of speed, to be indestructible and to have the ability to teleport. It is even said he is humanoid but with a different molecular structure than humans which gives him his fantastic abilities.
Well, really it was all in good fun. It was a meme created by Knudsen back in 2009 just for kicks but it took off like wildfire. Hopefully kids will stop taking it so seriously and just join in the fun in a safe way.