Over the weekend, 12-year-old American girls Anissa Weier and Morgan Geyser stabbed their friend 19 times in an effort to gain favour with a strange character they identified as “the Slender Man”.

Often referred to simply as “Slenderman”, the tall, faceless, long-limbed man the girls were referring to has been a popular figure on the internet for quite some time.

• Web character inspires young girls ‘to stab’

Born in an online forum, Slenderman began in 2009 as an internet meme after contributors started photoshopping him into the background of photos.

With an eerie presence and an invented penchant for stalking and killing young children, the Slender Man quickly gained traction online, going viral and inspiring hundreds of spin-off stories and myths.

The first known appearance of the Slender Man online: photoshopped into the background of an image containing young children playing in a park.

Internet users began spotting a narrow figure in the background of their photos, identifying the strange presence as the Slender Man himself. Others began photoshopping him into the background of their own personal snaps.

One of the main pioneers of the Slender Man craze is paranormal site Creepypasta, allowing users to share and collate their stories of run-ins with the creepy figure.

Anissa Weier and Morgan Geyser are reportedly big fans of the site and were inspired to commit their violent attack after becoming obsessed by online accounts of the figure.

Speaking to the Mail Online, Ms Weier’s brother William said his sister had become addicted to videos and stories of the Slender Man and spent her spare time researching him.

William said his sister’s obsession soon became so intense she struggled to separate dreams from reality.

“I know all about Slender Man. He surfaced a few years ago and the internet just seized hold of it and everyone started writing these Creepypasta stories,” Mr Weier said.

“It’s all just urban legends and Slender Man has become one of the biggest ones. Several people have made internet movies about him and there are online games as well. My sister never played the games, but she liked watching the videos.

“It’s been very shocking for our family, but what can we do?”

The girls are not alone in their obsession. A YouTube video, filmed by young boys and claiming to contain footage of the Slender Man, has received two million views online.