“Slenderman” by KaptynQerg at DeviantArt.

The Slender Man is a very young monster, having only come into existence in 2009, yet his notoriety is widespread. Slender Man is a popular figure in art, cosplay and fiction in the internet “creepypasta” tradition.

Slender Man is usually represented as an extraordinarily tall figure in a black suit and tie, with a featureless, bone-white face. He is often seen to have multiple long tentacles instead of, or in addition to, normal arms. He is associated with dark woodland areas and the disappearance of children.

The Supernatural episode #THINMAN (S09E15) is based on the legend of the Slender Man. In the episode, the Thinman legend was created by Ghostfacer Ed, but soon real Thinman murders start occurring in Washington state. It is one of the very few episodes, however, in which the “monster” is revealed to actually be a normal human (in this case, two men pretending to be Thinman).

Supernatural‘s Thinman and Ed the Ghostfacer

Ed: “It’s Scooby-Doo time, douchebag. Take off the mask. I know you’re not Thinman. You’re just a me-me.”
Harry: “Ed, it’s pronounced meme.”
Ed: “It’s spelled M-E-M-E though.”
Harry: “The second E is silent.”
Ed: “You’re a me-me. A man meme. And I invented you.”

One of two recovered photographs from the Stirling City Library blaze. Notable for being taken the day which fourteen children vanished and for what is referred to as “The Slender Man”. Deformities cited as film defects by officials. Fire at library occurred one week later. Actual photograph confiscated as evidence.
1986, photographer: Mary Thomas, missing since June 13th, 1986.
(Original Slender Man image and caption [#2] by Victor Surge from Something Awful Forums, 2009.)

Slender Man is the main character in several independent films, including the video series Marble Hornets. He is also the featured antagonist in the first-person alternate reality games “Slender: The Arrival” and “Slender: The Eight Pages.”

Due to the rapid creation and dissemination of legends of Slender Man as a creature born in the internet age, the lore surrounding him is widely various, but some of his powers are more frequently cited than others. These include the ability to teleport, and the transmission of  Slender sickness, symptoms of which can include nosebleeds, delusions, nausea, paranoia, and exhaustion. Effects of Slender sickness multiply and worsen each time The Slender Man is observed by the afflicted.

It has been postulated that The Slender Man is a Tulpa, or “thoughtform”—an idea made manifest through concentration of thought or belief. But just as there are no known weapons that are effective against Slender Man, Tulpas are also notoriously hard to kill. (There have been a few mentions of Tulpas on Supernatural: one appearance in “Hell House” [S01E17, also featuring the Ghostfacers]; and the muse Calliope was at first misidentified as a Tulpa in the show’s 200th episode.) With few other defensive options, the best tactic to adopt, should you see The Slender Man, is to run away, and run away fast.