Imagine parallel worlds. Places filled with gods and monsters both large and small, creatures that would terrify the normal person. Imagine these creatures hate each other. They want nothing more than to destroy each other and take over each other's worlds, until only one species remains on top.
Now imagine the only way they can get at each other, is through us.
This is because there are places on Earth where the walls between worlds are weak, allowing intruders to sneak through. This has gone on for millions of years, affecting us in ways we're unaware of, from stories to superstition to even evolution, and as time goes on, and the walls become weaker, the creatures become more impatient to attack our world and each other. Luckily there are heroes among us, a select few equipped to deal with this threat in ways unknown even to themselves. Without realizing it they’re drawn to these weak points, and thus to their destinies.