Setting. Three gods exist in the word: God of War, Goddess of Love, Goddess of Fertility. "Fight and die, since life is a cycle", "Love each other, since life need to start anew" and "Food and children, this is your survival of existence".
Death isn't final in the world. Clerics of the gods can resurrect, but only if the soul agrees. 99% time it doesn't, wanting to stay "with the gods", where "s/he's happy". The 1% wants to come back, but doesn't remember anything from his death time, even talking to the cleric.
One person dies and gets resurrected. He remember what he saw in the other world.
He stood with thousands of people. All screaming and yelling. Chained to the ground. Nobody can escape. He looks up to see three huge and hideous creatures, frog-like, giant hands, sharp teeth. One is covered with wounds and bleeds on the prisoners. Second is stabbed by many things, in constant pain. The last one, many-breasted, always lactating. Their hands move and scoop the prisoners to be eaten by their massive mouths. The gods are a lie. Demons in fake skin. During his resurrection he sees the entire plane from afar and sees what lies beneath the monsters and prisoners. Pillars, held up by hundred of thousand people. These are the faithful.
Some time after thinking about what he saw, he decides to change the world. Destroy the religion that binds the people. Save them. He takes control of the most militaristic city-state and goes on a conquer-spree. Soon the most faithful kingdoms stand against the "evil" one.
You can make the PCs part of the "good" kingdoms, where people follow the clergy and churches. Kings bow before statues of gods. A journey where in the end they'll know who is the "Man behind the Mask", why he wants to conquer the world and what is his solution the "problem".