(Most of this was taken from the Slender Man Wiki , and I edited the parts that do not apply to our reality)
The Slender Man is a being with various nebulously defined characteristics and abilities. Slender Man generally appears (in modern times) as a tall man in a black or grey suit, red or black tie, and white shirt, with no eyes, mouth, or clearly defined facial features, while in the middle ages until about the 1800's he was seen as wearing a cloak. It has no hair, and generally has normal-looking bare hands. Slender Man typically is depicted as between 6 and 15 feet tall, depending on the situation, and in video as around 6-7 feet tall.
In almost every description Slender Man's basic attributes will vary greatly, however several basic traits are typical of his behavior. He is shown to behave in a very passive aggressive manner, stalking targets for years at a time, torturing his target mentally for various unknown reasons. He is rarely ever shown in a benevolent light and is typically shown as a malevolent force.
The most important factor about Slender Man is the mystery. He lives unlike any human being despite having a similar appearance to one. It is uncertain if he is social, or even understands human languages or behaviors, nor why it is humans appear to be his main target. His targets will typically behave in off ways as if possessed, but whether their psychosis is causing them to speak for him or for themselves isn't entirely certain either.
Slenderman's appearance is variable as the years go on, but as of to date the most recent appearance is that of an abnormally tall human with long arms and no face whom is wearing a business suit. Again it is uncertain if his suit is real cloth or some form of skin molded into such an appearance. If it is the former, this would imply an intelligent being attempting to try to blend in. If it is the latter, it shows a being whom is extremely adaptive and evolves to match his environment and stick out less (or more).
Slenderman's appearance provokes a wide range of responses, the most common being fear. This is partially due to his extremely towering size over a full grown adult, which mimics the childhood fear of adult humans bigger than the child. His behavior also factors into it. Finally, the main fact is he is completely unable to be described in human terms. While a description in words can be given, human language lacks the ability to truly define the entity until such a time as when humans can interact with him (if at all possible).
In total two major types of Slender Man descriptions exist. The original versions depict him as not only malevolent, but extremely dangerous. He will do odd things such as removing organs and placing them in bags, impaling targets on trees, and aggressively stalking targets after a period of time. A contemporary depiction shows Slender Man as being extremely passive aggressive, often letting a person slowly delve into madness at their situation until being unable to cope. If invited to anger, he will typically charge a target down until caught and vanish with them to unknown locations. This version also shows an odd response to electronic equipment and causes massive problems with audio, surveillance, camera, and other various electronic devices with displays or audio. Sometimes an individual can determine if he is near simply by how certain electronics react such as radios, televisions, or cameras. How powerful this effect is seems to vary, and can either be due to Slender Man's own state of aggression, or other outside factors either inside or beyond Slender Man's control.
Slender Man's original abilities included invisibility to select people, adult humans, or anyone without use of a camera, the ability to change height and body shape, the ability to shape its arms into tentacles, and the ability to sprout tentacle-like appendages from its back and shoulders to be used as additional arms or legs (or simply to instil fear into its prey). Many stories seem to indicate that it can control a person's mind. Slender Man also had connections to children, with many of the original stories revolving around disappearing children or featured Slender Man being sighted in photos with children.
Most video involving the Slender Man contain some sort of video and/or audio distortion, indicating that Slender Man has some sort of effect on modern digital instruments. This does not appear in any of the original accounts- however these mostly make use of photographs and video of primarily lower-tech varieties.
Slender Man appears to also be able to teleport, an ability which is referred to in many videos as "Slender Walking." This ability is one of the few that is consistent with the original mythos about it.
Another consistent ability is being able to control or manipulate people's minds. A being under the control of the Slender Man is referred to as a Proxy .
Involvement with this RealityEdit
The Slender Man was first encountered by Answered in January 2013. Answered then created a group with the help of W-Man called the Slender Finders , with the purpose of trying to prove the Slender Man's existence. However, the group did not achieve much success as both founders were forced not to continue.
The Slender Man managed to abduct Answered shortly after he stopped, and has been trying to get, or at least drive insane, W-Man and the rest of the Slender Finders for an unknown reason. It is suspected that it wishes people who truly believe him in to perish, but nothing can be proved, for obvious reasons.
Shortly after Corrupt Secrets joined, the Slender Man abducted his sister and his best friend, probably as an attempt to get Corrupt Secrets to stop. It is also unknown why he did not simply abduct Corrupt Secrets.
There are many theories about the Slender Man regarding his existence and behaviour, but a new one has been introduced by Choices. This new theory is referred to as the Aspect Theory, saying that the Slender Man is merely an aspect of the Tall One.