Creepypasta's Slenderman: The Stuff You Need To Know About The Slenderman

In almost every description Slenderman'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. This behavioral pattern has an uncertain reasoning behind it, and exactly why he behaves in such a way has yet to be fully ascertained.
The most important factor about Slenderman 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 (Proxy), 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). This problem stems from the fact that he operates in a plane of existence further from humans. This is believed to be fourth dimensional space which gives him powers a human cannot understand due to humans living in three dimensions of space and being unable to comprehend the fourth dimension at this time.
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.

Abilities

Main page: Abilities of the Slender Man
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). 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.

As more stories came forth, however, the use of tentacles and tentacle-like appendages became less common, and the invisibility was limited or eliminated depending on the storyteller.
EMH, TT, ML, and DH all 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. EMH has applied this distortion to be the effect of Sigma Radiation, a radiation given off by Slender Man and the Rake. Sigma Radiation is also contagious, as it entered Evan's blood-stream when his arm is brutally maimed by The Rake
EMH and ML also make use of "Slender Walking", a term created to describe Slender Man's apparent teleportation, appearing, and disappearing at will. EMH independently indicates the use of selective invisibility to groups or individuals through head tilting, and also spacetime warping/teleportation between locations. Similar abilities were seen in early Slender Man accounts. EMH also hints at the affecting of mental health or control of it.

Canon History and Information

For any Non canon abilities and descriptions, please see Unconfirmed Slenderman Assumptions
Slender Man has no exactly defined or specified history, however contributors have placed early sightings of Slenderman-like beings in early 1600s Germany and before, where it took the dress of a knight or royal figure. Germanic fairy tales and mythology also makes use of the creatures to be used as cautionary tales for children.
Photographs from the early 1900s were the next confirmed reports, where imagery of Slender Man can be found in classical black and white and sepia imagery. Reports from this time indicate sightings in America, the UK, and Russia, as well as reports of child disappearances.
By the mid-1900s, some rare run-ins occurred in the war zones in Germany, apparently Slender Man's native land. Soldiers were the primary targets here. In America and Canada, reports of missing skiers and children appeared, coming from forested areas of the nations.
After a spree of building burnings from unknown causes, several deaths and a few surviving witnesses, it could be gathered that the Slender Man had the following traits:

Slender Man will find interest in a victim for reasons unknown
It would then contact the victim, if it is a child, presenting itself as friendly
The adults he stalks have a common trait: they have all been through a terrible tragedy in their life, even if the tragedy was made by Slender Man directly
If an adult, it would stalk the victim for long amounts of time causing what is known as "Slender sickness" causing massive paranoia, nose bleeds, nightmares, hallucinations appearing to only the sick person, and many other dangerous symptoms.
Eventually, it would abduct the victim into nearby forest, where they would be killed.
In 'messy' cases, it may remove evidence of its existence by causing fire of their home, place of work, or school

Death of the victim originally occurred through an odd means- the victim would be impaled on branches of very tall trees while still alive, and bleed to death. The organs would be individually removed, then placed in plastic bags. They would then be replaced into the body in their original positions, plus the plastic bags. The victims' corpses do not show any sign of a struggle. As time has gone on, this practice has seemingly been abandoned.
In the late 1990s, technology shed more light on the creature through the use of commonplace cameras and camcorders. Slender Man was recorded and photographed stalking potential victims many times.
In the late 2000s to early 2010s, several individuals recorded ongoing stalking and attack scenarios of themselves and their friends. These video accounts are codenamed Marble Hornets, EverymanHYBRID, DarkHarvest00, MLAndersen0, and TribeTwelve. As well as certain Rivn videos.
The new foray into Slender Man's powers, abilities, and attacks have shown drastic difference from his original behaviors. The original desire to impale targets on trees, his rather aggressive attitude, and attacking mainly children have been altered. It appears he seems to enjoy targeting humans who have seen him in childhood and following them for years. He appears to do so in such a way that the target is unaware of his presence for potentially months, years, or even decades, and typically won't be aware at any point in time until it is far too late.
His behavior is extremely passive aggressive, where he stalks, follows, and torments an individual by repeatedly showing his presence. He will typically do so in invasive ways, such as appearing inside someone's house, outside their home, on roads they travel, at their place of work when they are alone, or while the target is sleeping. This will create a general unease of the target, as the invasion of an entity in their own safety zone will begin sparking incredible paranoia. Most often he won't attack the target for extended periods, instead just following the target and tormenting them. This period can last anywhere from several minutes to years.
The behavior of taking organs out or impaling targets have been long forgotten. If his new modus operandi is to be believed, the target is taken away with no traces whatsoever. The target will never be found, nor are any traces. Thus his new method of killing targets is unknown, as no victim is ever found. The victim will typically

Untitled — Slender Man at the bottom of the stairs in EMH

disappear by him/herself, thus these attacks are unrecorded and unknown. What little camera footage has been found shows incredible distortion on them in typical cases, making it nearly impossible to understand exactly what he has done. To date it is unknown where his victims go, or what he does with them after this. One of the most powerful new traits Slender Man will typically employ seems to be some form of inhibiting memory loss. Like Slender Sickness, this effect can be nearly immediate just on close contact with him, although it seems to vary as to exactly how powerful. In some cases individuals will forget the past few minutes, however extended contact has led people to awakening in locations they are uncertain of due to running from Slender Man and their brain beginning to write memory and restore consciousness after a given period of time. In some cases, it is believed Slender Man himself will deposit a victim in an unknown location and leave them to awaken after an attack (possibly to continue stalking them some more). This effect is akin to Slender Sickness, but is the most powerful effect of the sickness to date.
Victims of extended periods will note excessive damage to their social, physical, and career life. Typically individuals will show massive moods swings and constant irritation, not to mention will lose track of large periods of time, making communication with friends or holding down steady work difficult, as memory losses could be as long as several weeks or months until the individual awakens and attempts to return to their life only to find it ruined. Despite this, Slender Man will continue his behavior.