Sometimes this world slips and one of those other worlds that lie on its edges slips through. This doesn’t usually matter that much for it is the nature of these worlds, these universes, that edge against each other that they are very similar. They change more the further they are from one another, until even a pair of worlds a few hundreds apart start to show significant differences.
However, there are those creatures that have evolved over the eons that can slip between these worlds with relative ease. Some of them come from many, many, parallel universes away, travelling as easily as a modern urban human can travel across a city.
In the past, these creatures were demons and devils, monsters and even aliens. All Earth cultures have their tales, myths and legends about these strange creatures that come out of the darkness, out of the shadows. Some, for a while after the old religions faded, thought the tales of these strange beasts haunting the edges of our lives would fade too, become little more than a tale to tell to children.
These creatures, though, did not fade away into the worlds of myth and legend. People never stopped telling tales of strange happenings out on the edges of civilisation where the darkness can hide so much.
Then came the slippage, a sort of earthquake where the fault lines of these parallel worlds rub against each other. Suddenly, these creatures found what amounted to a shortcut between their worlds and this one.
So, nowadays everyone stays away from those dark shadowed places, leaving the night to these predators that lurk, waiting to pounce. Some are even beginning to say that this world, this Earth, no longer belongs to us humans and it now belongs to them, the creatures, while we are now little more than their prey.