Slenderman director on the 'faceless' character's fascination

"Most kids in America know who Slenderman is," says Irene Taylor Brodsky, director of Beware The Slenderman.

You may also know the tall, willowy, faceless - but well-dressed man - who lurks in the background of photos.

Taylor Brodsky says "most kids" know he's not real but two 12-year-old girls became immersed in Slenderman's world.

The pair from Wisconsin admitted stabbing their friend 19 times after Slenderman "told them to".

The film - which premiered at the SXSW festival in Austin, Texas - follows the 18 months after that happened.

In June 2014, Morgan Geyser and Anissa Weier calmly told police how they had plotted to kill their classmate.

Slenderman is faceless and I think for me that is the most captivating aspect of him
Taylor Brodsky

They said if they didn't, Slenderman would murder their families. The girls' reward was to live as the tall man's servant in his mansion in a nearby forest.

The HBO film shows their initial police interviews, court appearances and also looks into why this character has become such a cult internet figure.

"Slenderman is faceless and I think for me that is the most captivating aspect of him," Taylor says.

"We can project our own discomforts onto his face.

"I think children have a very dual idea of what Slenderman can be, he can either be this ruthless killer or he can choose special children and protect them, so he is a guardian angel as well."

The Slender Man character is often the subject of online horror stories written by teens

This could be reason for Geyser and Weier's obsession with him. The documentary suggests the girls didn't have many friends at school.

"When we talk about believing in things, these girls tell us they truly believe in Slenderman and that he existed," says Taylor Brodsky.

"When you believe in something, it makes you feel like you are a part of something and I think that is one of their motivators for belief.

"If these girls had a very robust group of other girlfriends, Slenderman may not have played such an important role in their lives."

That is not to say there is any excuse for what they have done, says Brodsky.

premiere

The film also touches on Geyser's schizophrenia diagnosis.

She is now being treated at a state mental hospital, where she is said to be making "substantial progress".

The girls' parents are also interviewed in the documentary.

Along with the parents of Anissa Weier it is clear - as Taylor Brodsky puts it - "they are completely perplexed".

She says they are crushed by what happened.

Anissa's father Bill Weier appeared at a talk at SXSW festival and was asked why he thought his daughter was so fascinated by Slenderman.

He said: "I couldn't begin to tell you."

Weier and Geyser are both awaiting trial. They are currently appealing against a decision to keep their case heard at an adult court rather than a juvenile court.

This could mean the difference between being released after 18 or 45 years in jail.

premier

Beware The Slenderman has already drawn comparisons with other real life crime dramas, such as Making A Murderer and The Jinx.

But as Brodsky points out the big difference with this film is that it is not a "whodunnit".

"We knew six hours after the stabbing occurred that these girls had done it.

"I think what the film has become is really more of a story of the delving into the how and the why, these are all important questions we need to be talking about."

Morgan Geyser and Anissa Weier
Image caption Morgan Geyser and Anissa Weier

Find us on Instagram at BBCNewsbeat and follow us on Snapchat, search for bbc_newsbeat