The Internet doesn’t just help suspend disbelief. It rolls right over it.

Exhibit A: Two 12-year-old girls from Waukesha, Wisconsin, charged with attempted murder for stabbing a friend 19 times and leaving her for dead. (She miraculously survived.) They appeared in court Wednesday.

The savage crime attracted international attention not only because of the age of the alleged perpetrators and the barbarity of the deed but also for something far more bizarre. The stabbing was apparently triggered by an Internet-generated fictional character named “Slenderman,” a sepulchral figure with long tentacles who kidnaps children and, by the girls’ accounts, requires acolytes to commit murder to be admitted to his realm. In trying to kill their friend, the girls said, they were attempting to appease the Slenderman so they could join him.

Commentators shocked by the crime have labeled Slenderman a new iteration of urban myth, one of those horrid tales that emerge from the collective consciousness. Stories like the woman who had a nest of black widow spiders in her beehive hairdo and died of a bite; the man who awoke to discover one of his kidneys had been removed; the psycho axe-murderer who lurks on lover’s lane, or the ferocious killer alligator that lives in a city’s sewer system.

But there is a key difference between Slenderman and his mythical forbears. He exists at the juncture of urban legend and the Internet, and the Web has introduced a powerful new element into urban mythology. It has so commingled fact and fiction, the real and the fabricated, that two young girls were allegedly willing to kill for their conviction in the authenticity of Slenderman. They had, in effect, entered a chilling, alternate reality.

Urban legends have been around for a long time, and a few, like Slenderman, have attained a certain credibility — even when they seemed far-fetched. It is as if people wanted to believe.

There are reasons for that desire. These stories can provide the same sort of creepy adrenaline rush as horror movies or Stephen King novels. Since many are collaboratively constructed, like folktales, they give us all an opportunity for — and pride in — authorship. Perhaps most important, they amplify our social terrors by displacing them into these narratives.

That is certainly the case with Slenderman, who embodies our fears that our children are forever endangered, and that there is nothing we can really do to protect them from a host of perils.

At some level, we suspect these nightmares are a projection, not a reality. We suspend disbelief — and we know we are doing so. Not too many people, however, subscribe enough to a myth to be willing to kill for it. That takes legendary to a whole new level.

In the wake of the tragedy in Wisconsin, some psychologists commented on the girls’ possible mental instability and on the extent to which their minds might have been colonized by Internet imagery — the way earlier generations had been colonized by novels and movies. But few of us were unable to distinguish between the page and the palpable — or between the screen and the substantial.

Because we didn’t have the Internet.

It is a commonplace that the Internet has no gatekeepers the way traditional media have. There is no filter on information and in which fictions can be captured before they reach the national conversation. All sorts of effluvia emerge on the Internet. It is basically an open sewer of information out of which we are personally entrusted with determining truth.

What this has done is change our perception of truth and muddled it. Which is why urban legends can now assume a reality. (It is also why even the most outrageous political anecdotes assume a reality too.)

You think of the Internet as a way to connect to the world or purvey information or facilitate collaboration. The site on which Slenderman gained notoriety is “Creepypasta Wiki,” and it allows visitors to invent, revise and embellish stories about the mysterious figure — which meant that they had, in various ways, internalized him.

But the Internet is also a demonstration of what biologists call “swarm theory.” The basic idea is that a swarm of bees, say, has a collective consciousness that is larger than the consciousness of the individual parts or members of the swarm. This mega-intelligence is what enables a swarm of bees or a colony of ants to act as a single, unified entity.

The Internet may very well be to humans what hives are to bees. The collective consciousness created by all who contributed to Slenderman, for example, seems to be larger than the individual contributions. It is not only a separate thing — it creates a separate reality with its own gravitational pull. When things enter the giant maw of that consciousness, one may be unable to detect what is real from what is not. The old lines of segregation melt. We either join or enter a mass mind. We lose ourselves.

So when two susceptible young girls read about Slenderman online and seek, out of some strange, misguided impulse, to join him, they may be caught up in the swarm. Indeed, one could say that the girls really weren’t trying to appease Slenderman, as they claim, so much as they were trying to pacify the Internet itself and the new reality it creates. This also makes the poor victim a casualty not just of deranged “friends” but of a deranged notion of reality — in which Slenderman is an actual creature.

We live in the age of the Internet. But you can die in the age of the Internet too — because it is so far larger than we are and it possesses its own world.

Just ask anyone who believes in Slenderman.

PHOTO (TOP): A screen shot from “Slender: The Arrival” video game shows the Slender Man. Reuters/Blue Isle Studios

PHOTO (INSERT): A colony of honeybees swarm on the ledge of a window outside the Media Center, in Bern June 17, 2013. REUTERS/Ruben Sprich