What is Slender Man?

The creepypasta meme known as Slender Man has only been around on the Internets for 3 years now. It was spawned June 10th, 2009 for a paranormal pictures Photoshop contest on the Something Awful forums. SA forum user Victor Surge was the first person to post Slender Man pictures and coined the term "The Slender Man," which has since turned into just Slender Man.

Here's Victor Surge's first post from June 10, 2009:

"We didn't want to go, we didn't want to kill them, but its persistent silence and outstretched arms horrified and comforted us at the same time..."
1983, photographer unknown, presumed dead.

"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.

Here's Victor Surge's Second Slender Man post dated June 11, 2009.

5/24/95**
1994: Wilks Estate. One subject reported nothing out of the ordinary before taking photograph. Lower stairs area was said to be very dark. Subject states that after the camera flash she heard a sound like a watermelon being *unable to understand subject*. 
5/25/93**
Subject unable to recall events after manor power failure. Unable to question other two identified subjects. Camera and film acquired from Gloria Cready, current resident of Woodview Mental Hospital and Psychological Rehabilitation Clinic. Film mostly uncontaminated despite mass of blood and human tissue present on camera. No positive ID on anomalous tall and slender subject. Facial blur caused by possible contamination. 
6/7/93**
Early digital analysis indicates tall subject may have no eyes. Anomalies, previously thought to be film errors and flash artifacts, now thought to be appendages. 
6/10/93**
Final identified subject reported missing along with other thirty-three patients and staff of Woodview Mental Hospital and Psychological Rehabilitation Clinic south wing. 
6/18/93**
Further inquiry to cease immediately.(see report No.3339-2)

June 12, 2009, Victor Surge's Third Slender Man post:

Slender Man at Jake's Birthday party.

Victor Surge's Fourth Slender Man post is where the mythos of Slender Man really starts to take shape. Victor then went on to say that Slender Man was inspired by "The Rake" meme. 

Steinmen Woods

Both subjects were hunting in the Steinmen woods four hours before sundown. Surviving subject states that while hunting both men grew uneasy as fog levels rapidly increased. A constant murmuring sound accompanied by a low hum eventually became apparent to the two men an hour after the fog increased. An object falling out of tree stuck one of the men in the left shoulder causing him to discharge his weapon. Object said to be the body of a man of unknown age. It was very precisely dissected, with major internal organs still contained within the rib cage in what looked to be clear bags. Surviving subject placed organ bag within backpack. Attack followed several minutes later after a "low children's laugh, like a giggle". Surviving subject ran until he reached his vehicle. Subject then drove to assumed safety.

Backpack destroyed.

Surviving subject is classified as a B7 witness. B7 witness to be placed in quarantine "Blind Box" until resolution. 

2007:Investigation team discovered twenty-two bodies of both genders and various ages impaled on broken tree branches in a radiating circle pattern with chest mutilation as often noted with Slender Man. Upon confirmation, lead investigator ********* called for an immediate evacuation of investigation team at 1700 hours.

Bodies first discovered at 1100 hours. Deadline for safe evacuation of team with only viewed physical evidence of Slender Man approximately 1730. Lost contact of team at 1725. Safety procedures fell well within established protocols. Reason for abnormality is unknown. Second team recovered camera equipment one week later. Slender Man safety procedures require this incident's physical photographic evidence to be disposed of by no later than 10/20.

I honestly don't get what half this poo poo means. I'm done with this Slender Man stuff. It's starting to make me uneasy. It's like reading the GBS ghost story threads before I go to bed. Why do I have to look at this stuff while it's super late?

Luckily, my friend is coming over.

"My friend is herejus camein barely made up staairs got pictur locked door but it s right there inthe hall dont look at its pictures it dosent want to be known about dont loo"

(The "Don't Look" at the end of the post appears in the video game "Slender" which is based on the posts by Victor Surge and takes some of its ideas from "Marble Hornets" Slender Man videos on YouTube.) 

Soon after these posts on Something Awful forums, Slender Man went viral. By the end of June 2009, the YouTube Channel "Marble Hornets" had appeared. There's even a  Slender Man Wiki now. Check out this newspaper clip from the wiki.

Since its inception, Slender Man has morphed from a child abducting monstrosity, to an alien, almost fae-like entity that steals people and takes them away to its home in the woods. Like the fairy lore of old, Slender Man can warp distances-- such as turning a small home into never ending maze-- and affect the passage of time. It can stretch to reach impossible heights and in some works, it has tentacles that sprout from its back that it uses as extra arms or legs. 

In newer versions of Slender Man it seems to be able to control a person's mind and can teleport to appear in front of people that are running away from it.

In Marble Hornets, the appearance of Slender Man is marked by video and audio distortion. 

The visual aesthetic of Slender Man, with the black suit and long tendril fingers or tentacle arms, has struck a primal chord of fear with many people that has caused it to become one of the scariest urban legends of our time. Now there are people that believe that Slender Man has become a real entity in our world due to the "Tulpa Effect"-- if a group of people believe in something strongly enough, like a god or a monster, the energy from their collective minds, their psychic power, makes it becomes real. Of course, whether or not such a fantastic phenomena is possible is still up for debate. 

All I know for sure is that Slender Man meme is great creepypasta.