I just finished looking over the article, and I think that it basically meets good article criteria! I have a couple of notes, but overall there isn't much to complain about. The copyright section was especially interesting. I wonder who the mysterious third party option holder is! Also, I'm curious, do you think there's a case to be made to add the two original Slender Man images to the article based on the WP:NFC policies?
The wording of these sentences is a little confusing: "The Slender Man was created on a thread in the Something Awful Internet forum submitted on June 8, 2009, with the goal of editing photographs to contain supernatural entities. On June 10, a forum poster with the user name "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." it might be better worded as, "The Slender Man was created on the Something Awful Internet forum on June 10, 2009 for a photoshop contest whose goal was to edit supernatural entities into photographs. A forum poster with the user name "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."
"His intention was, he claimed, 'to formulate something whose motivations can barely be comprehended, and [which triggered] unease and terror in a general population.'" - The quote from the source says, "I used these to formulate something whose motivations can barely be comprehended and causes general unease and terror in a general population." Is there a particular reason why "causes" was changed to "[which triggered]"?
"As of 2013, Marble Hornets is now one of the most popular Slender Man creations, with over 250,000 subscribers around the world, and 55 million views." - I didn't see anything in the source about Marble Hornets being one of the most popular Slender Man creations. Is it located in a different source? If not, it might be better to rewrite it to say something like, "As of 2013, the Marble Hornets YouTube channel has over 250,000 subscribers and 55 million views."
"In 2012, the Slender Man was adapted into a video game titled Slender: The Eight Pages; as of August, 2012, the game has been downloaded over 2 million times." - The quote from the source says, "In fact, it’s so good at what it does that in little under a month the game has been downloaded over 2 million times..." 2 million times in a month seems pretty significant; maybe it could be reworded to say something like, "In 2012, the Slender Man was adapted into a video game titled Slender: The Eight Pages; in less than a month, the game was downloaded over 2 million times."
This bit, "The success of the Slender Man "legend" has been ascribed to the chaotic, ambiguous nature of the Internet. While nearly everyone involved understands on some level that the Slender Man is not real, the Internet offers up a mess of conflicting perspectives, blurring the boundary between fiction and reality and obscuring the character's origin, thus lending it an air of authenticity." should probably be attributed to the author.
Ok, I just finished the lede. It's a bit short, but I don't think that's necessarily a problem because it does a really good job covering the Origin and Description sections. There are two things that I think would really improve the lede to make it a true overview of the body. The first is a sentence or two mentioning some of the works that Slender Man appears in... The three that seem the most deserving of a mention to me are Marble Hornets, the Enderman, and Slender: The Arrival. The second thing that I think is important to mention is the Slender Man-related attacks. I also think it would be nice to add maybe one sentence about Slender Man as folklore, but I don't think it's necessary.