how does it get its name?
Because of his  tall, thin, spectral figure
Slender Man
The Slender Man was created on June 10, 2009, on a thread in the Something Awful Internet forum. The thread was a Photoshop contest in which users were challenged to "create paranormal images." Forum poster Eric Knudsen, under the pseudonym "Victor Surge", contributed two black-and-white images of groups of children to which he added a tall, thin, spectral figure wearing a black suit. Although previous entries had consisted solely of photographs, Surge supplemented his submission with snatches of text—supposedly from witnesses—describing the abductions of the groups of children and giving the character the name "The Slender Man":
when did slender man the movie come out August 10, 2018 was the movie based on any incident? No - Slender Man  is a fictional supernatural character could you please describe him? Thin, tall humanoid with a featureless head and face, wearing a black suit. by whom was it developed? Eric Knudsen is it related to any stabbing incident? Slender man is not related to any stabbing incident. However, Slender Man readers were connected to several violent acts, particularly stabbing of a 12-year-old girl in Waukesha, Wisconsin. where is the latter place located? Wisconsin, United States.
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Slender Man [SEP] Origin
The Slender Man was created on June 10, 2009, on a thread in the Something Awful Internet forum. The thread was a Photoshop contest in which users were challenged to "create paranormal images." Forum poster Eric Knudsen, under the pseudonym "Victor Surge", contributed two black-and-white images of groups of children to which he added a tall, thin, spectral figure wearing a black suit. Although previous entries had consisted solely of photographs, Surge supplemented his submission with snatches of text—supposedly from witnesses—describing the abductions of the groups of children and giving the character the name "The Slender Man": The quote under the first photograph read: The quote under the second photograph read: These additions effectively transformed the photographs into a work of fiction.