Based off a novel I wrote in the early aughts. The modern world is not as it seems - reality itself can be altered by willpower, though only a tiny minority of the population is even aware of this. Such individuals are known as mindbenders.
Parallel worlds (commonly referred to as underworlds or realms) are overlaid on our own, invisible to most people yet interacting with our own. In ancient times, the inhabitants (commonly known as denizens) of these alternate dimensions wreaked havoc on our own, and are the source of many of the myths of gods, fairies, demons, and monsters. Mankind sought to free itself of these threats, first by establishing religions (masses of people believing something helps to shape reality, making it more difficult for monsters to influence our world), and more recently establishing science (limiting the influence of these other worlds).
Though not all denizens of the underworlds have ill intent, many resent humanity for these restrictions, and a hidden war rages about as they seek to gain more freedom and influence over our world. Among humanity, only mindbenders are even aware of this and serve as front-line troops in this constant struggle. Though our world cannot physically interact with the others and vice versa, mindbenders and all creatures from other worlds can peer through the veil and see other worlds. With a strong enough will, they can even influence those worlds, projecting their minds into avatars to physically interact with them.