There are three big Slender Man series on Youtube: "EverymanHYBRID," "TribeTwelve" and "Marble Hornets." Each series manages to terrify the audience in its own unique way, and each has added a little bit more to the Slender Man mythos to establish a horror setting that I like to call the Slenderverse.
"EverymanHYBRID" is a Youtube series that began on March 21, 2010. It's about three dudes that started a cheap fitness video series; Jeff, the lanky camera man who mumbles; Vincent, the rational, charismatic Face of the group; spunky Evan, who can't remember what a thumb drive is and is always ready for a fight.
According to the Slender Man Wiki, they originally intended to create a fake Slender Man and put it in their videos, but after Episode 6, the real one shows up at Evan's house and starts stalking them. Slender Man (or one of its agents) has left behind a trail of clues for them to follow as it torments them. Soon after the guys start investigating and trying to figure out a way to stop Slender Man, other types of creepypasta creatures, such as the Rake, begin to crawl out of the woodwork.
I like to think of "EverymanHYBRID" as the Scooby Doo of the Slenderverse, as the guys investigate not only Slender Man but other monsters from the Internet as well, which creates complex set of side stories and subplots for the series. The subplots revolve around strange supernatural activity experienced by family members and friends of the main cast. These episodes are more like stand alone story arcs, much like a TV series will have a main plot (or metaplot) and episodes that aren't related to it that serve to develop the character's private lives and backgrounds.
"EverymanHYBRID" is an alternate reality game that's very audience oriented. The "EverymanHYBRID" wiki has a suggested viewing list that contains response videos and pictures of items that people have found. The main "EverymanHYBRID" channel does not have these extras listed, which is unfortunate, because without the additional clues and video responses to fill in the gaps of information present in the main videos of the series, things get confusing rather quickly.
Overall, "EverymanHYBRID" isn't very subtle and at times it has a problem with creating a suspenseful atmosphere. My biggest complaint is that it has an extremely slow start, and it really doesn't start to feel like a horror series until the video "Ashen Waste." The video "Alex" is when it really starts to get good, as it is far more subtle and leads to more scares and highly suspenseful sequences than the previous videos in the series. Ironically, this is also a side story episode and not a part of the main plot.
Also, I should warn you, the boys in the videos drop F-bombs like you wouldn't believe-- making it unsuitable for kids, and in some instances, not safe for work.
The "TribeTwelve" channel originally was a Youtube page for Noah Maxwell's college project on the Twelve Tribes of Israel, but the project was suddenly cancelled by his teacher and he had to scrap it.
When Maxwell's closest friend and cousin Milo commits suicide, he decides to turn his Youtube page into a memorial and post videos of Milo's last visit with him. Soon afterward, he realizes that the videos of their time together reveal that something sinister was following Milo. Now it's after him.
The "TribeTwelve" Slender Man series began June 4, 2010, several months after "EverymanHYBRID" started being posted on Youtube.
"TribeTwelve" has some videos that are set up similar to "Marble Hornets," with text introducing the video to create suspense and tension in the viewer. The video is also in the found footage format, with the camera filming from the point-of-view of the main character.
There are some similarities to "Marble Hornets" and "EverymanHYBRID" to be found in "TribeTwelve," such as mysterious packages, letters and messages left on the Internet or mailed to the main character, and specific haunted locations in the woods. "TribeTwelve" also borrows a lot of visual and audio cues from "Marble Hornets," but it clearly uses its own visual aesthetic for the distorted imagery that occurs when Slender Man gets near a camera.
This series uses an Internet meme about an urban legend called "Der Grossman;" a German fairy tale about a tall man that lived in the forest and hunts children. The video "My Grandfather Karl" tells the tale in German and provides a genuine feel to the "TribeTwelve" series that rather I quite enjoyed.
I should warn you, the main character of "TribeTwelve" loves to drop the F-bombs, but not as often as his foul mouthed counterparts in "EverymanHYBRID."
1. Marble Hornets
Jay's friend Alex was working on a movie called "Marble Hornets." After two months of filming, Alex abruptly dropped the project and moved out of state. Jay hated to see all of his friend's work go to waste, so he convinced Alex to give him the tapes. When he started watching them, he discovered that Slender Man (aka The Operator) was stalking Alex and had broken into his house on several occasions. Jay started the "Marble Hornets" channel and began posting the videos of the strange occurrences that Alex video taped.
Now, Slender Man is actively hunting Jay while its agent "totheark" delivers disturbing packages and leaves him taunting cryptic video messages on Youtube. Jay always leaves a link to the video responses made by "totheark," which makes it easier to follow the trail of clues left for him. He also goes out of his way to film the packages that are sent to him so that the audience gets a good look at their contents.
The "Marble Hornets" series was first posted June 20, 2009, several months after Slender Man was created on the Something Awful forums. The plot line and found footage format of "Marble Hornets" was inspired by SA user Nashie O, who mentioned that Slender Man needs a movie that is a collection of documents and sightings like "The Blair Witch Project," only better. Clearly, the guys that made this series listened to what the creators of the Internet meme had to say, as it is wonderfully subtle when creating the suspenseful scenes that lead up to the appearance of The Operator.
The Slender Man practical effects in "Marble Hornets" are simple, but very well done. The puppet combined the forced perspective creates a nightmarish monster that can change its height at will.
"Marble Hornets" is the grand daddy of the Slender Man video tropes-- it helped establish that video and audio distortion signals the approach of Slender Man, and it created the idea that Slender Man's victims suffer from coughing fits and severe paranoia. "Mable Hornets" has also established some of the more iconic setting pieces for Slender Man, such as the old factory with rusted out steel fuel tanks and a fort tower in the woods, to create vivid and terrifying imagery that the other Slender Man series and video games are now attempting to duplicate.
"Marble Hornets" also created the idea that Slender Man turns some of its victims into a puppet or herald that works as its agent. The agent in this series wears a simple black and white mask that conceals his identity. Personally, I think that the masked agent is "totheark," however fans like to call him "Masky."
"Marble Hornets" is the scariest low budget horror series I have ever seen. It perfectly sets up the terror early on, and never fails to deliver, making Slender Man a truly terrifying monster to behold. This series scares the bejeezus out of me every time I watch it. It's so good, it gives me goosebumps. I'm not kidding. This series is creepy as hell.