Slenderman is a very tall man. He is pale and lanky. He appears to have tentacles that come from his back and two main arms that look human with very long fingers. Slenderman’s most frightening attribute is his lack of a face.
The Tulpa Effect is a theory on the creation and existence of Slender Man.
"Tulpa is a concept in mysticism of a being or object which is created through sheer spiritual or mental discipline alone." - Wikipedia
A Tulpa is a thoughtform, or being created from the collective thoughts of separate individuals. The concept of Tulpas is theoretical in nature and originates from Tibetan mythology, where Tulpas are described as extra bodies that were created from one person's mind in order to travel to spiritual realms.
The Tulpa Effect is the name given to the unintentional creation of a Tulpa based on collective belief of a being with similar traits.
The Slenderman page says, “the idea of intentional creation of Slender Man as a tulpa came as early as August of 2009, as the idea was first thrown around in the Something Awful forums.”
The idea is still carried today by some Slender Man-based communities, specifically Slender Nation, although the various separate evolutions of the Slender Man creature have created varying beliefs, traits, descriptions, and actions, and no longer includes a single unified description of Slender Man from which a tulpa could be created.
Core Theory centers around the idea that the Slender Man was created as a tulpa in some way in the various ARGs, and acts as an in-game reasoning as to its existence and slight differences among the various series.
Some have worries over the unintentional creation of a Slender Woman tulpa due to the popularity of the mythos in present day. Others believe that the Tulpa Effect may have serious real-life repercussions with the creation of a real Slender Man.
Slenderman shares the same traits as his supernatural cousin, ‘The Rake‘: “During the summer of 2003, events in the northeastern United States involving a strange, human-like creature sparked brief local media interest…”
There seemed to be a number of people who were seeing a bed-side entity that was being called “The Rake.” The name was taken from an Internet meme that was created on a social network. The Rake was a hairless creature reported to be at least 6 feet tall, emaciated and sinewy with dark black eyes. The entity allegedly looked like a balding old man. The ribs could clearly be seen against the skin and the lips on the face were pulled back slightly giving the entity a death grin.
People lost interest in the Rake stories of the northeast after Slenderman appeared in 2009.
Both the Rake and Slenderman have similar traits to another supernatural entity known as the “Grinning Man.”
A website devoted to the grinning man tulpa is ThisMan.org, where the site asks if anyone has had dreams about a bald nondescript grinning man. The website is a dream registry that seems to show that this grinning bald man or the slender man without a face is now in collective unconscious.