A 20th-century Earth that looks like any other, and a second world parallel to it. In each of the thousand fronts of a millennia-old conflict, the forces of one fight to keep the worlds distinct, while the other tries to join them together. On Earth, sorcerers fight alongside androids, shrine maidens and monster trainers, united in a desire to keep their war out of the eyes of mundane powers. From the Other Side, demons and necromancers, harpies and ogres, slimes and sentient plants fight to join the two and expand into the world of humans.
At least, sometimes, that's how it is. While some humans have noble goals, others are in it for the opportunity to use their powers for adventure and profit by beating up seemingly unambiguous villains and jacking their stuff. A loose conglomeration of Earth-based organizations only very barely keep tabs on these people, seeking to avoid unnecessary panic and conflict and to share vital information.
While some non-humans are zealous in their attacks, just as many don't care about the war. The most noticeable of these make up the Merchant Cabal, explicitly out to make the biggest profit possible. Yet others are far more concerned with fighting each other for power, vengeance, or any other imaginable reason, to the point that some Dark Lords don't mind working with humans, if that's what furthers their interests.