Web creation Slender Man 'told' girls to attack 12-year-old friend in US

He lurks in the background of gritty black-and-white photos - a gaunt, too-tall figure with skeletal limbs.

Some say he lives in the woods and eats children, a kind of demon descended from eastern European myth.

Some say he stalks human prey indiscriminately, wherever he can find it: in basements, outside half-open windows, along lonely streets late at night when only occasional headlights cut across the road.

Some say he has no face. Others, that his face looks different to everyone who sees it.

Whatever they say, though, everyone generally agrees on two points: that Slender Man may be the internet's best and scariest urban legend, and that he is utterly fictional.

But that did not stop his legend almost resulting in the death of a 12-year-old girl in the United States, when two other girls her age plotted to kill her to curry favour with the character.