A world where dreams and things believed, the collective consciousness of the people, can become real and walk the mortal realm.
Deities, long imagined by mortals crying for help in dark times, live in Nyx, the star-filled sky that makes up the material of once imagined, now created things. The deities fight and work towards gathering worshipers, increasing their powers. Trying to interfer in the mortal realm, they seek champions to do their bidding and spread their beliefs, gathering more people towards their cause.
The world is made up of three layers:
Nyx - the sky and everything beyond the edge of the world. where gods and powerful creature live, only the most daring, some say suicidal, adventure.
Mortal Realm - the land mortals live in, filled with barren lands, lush forests, mysterious island, and polises, city-centers where people gather away from the monsters beyond.
Underworld - Only the dead and those who watch over them live here, although a few have been known to escape the tight grasp of Erebos, God of the Dead.
Based on classic Greek/Roman mythology
Ref:
Mythic Odysseys of Theros, Wizards of the Coast