The Divided Kingdom is 300 meets bible history. Combat will be cinematic and brutal.
The idea of having fantastical creatures is explained away by the characters meeting Assyrian, Egyptian and far east creatures that are so foreign that they appear to be fantastic creatures of mythological notoriety.
Magic and other powerful supernatural acts are rare, but not unheard of and people fear those who use them; sometimes worshiping them as gods, and sometimes stoning them the death as devils.
Demons are real and active. They pursue humans in an effort to gain power and influence over the world, so they often seek individuals with the potential to be influential or who are already in positions of influence.