Building on the "sun god as evil" concept:

The campaign world exists in a binary system, and has two suns. The main sun is visible during the day, and is very powerful, leading to the arid, desert climate of the world. Outside of civilization, the days are unsafe- many types of monsters and evil beings who thrive in the heat roam the wastes and attack any humanoids they find. Obviously, this is the evil sun god.

When the main sun sets, the second, weaker sun rises, giving way to this world's equivalent of "night"- a sort of 12-hour twilight, like those old movies that simulate night shoots by using blue camera filters. The secondary sun emits some sort of radiation or light ("magic", as far as the PCs are concerned) that drives the monsters away and makes the land safe for travel. Since the sunlight is also weaker, this is the main period of activity for the world's inhabitants, who conduct all their work and business at night and then rest and sleep underground during the day. Obviously this sun is the good, life-giving variety.

The orbit of the planet is not such that the day and night are always evenly divided. Every so often, one sun will intrude on the other's domain. This could give rise to the occasional night when the monsters are still able to come out and attack the cities, or the occasional day when the radiation of the weaker sun drives them away. Either one can be interpreted as the good and evil sun battling for dominance. So the church could have these as holy days, crusades could be declared, etc. The problem is, adding a second sun into the mix, even if it is the weaker sun, still isn't going to cool things off. So these days of battle would be especially unbearable and exhausting.

Major civilized areas would probably be in temperate zones, with an inhospitable equitorial zone in the middle. Perhaps there are two main civilizations in the world, in northern and southern hemispheres, and they are seperated by their inability to make it across the tropics. There might even be some way to work things so that each polar cap is bathed perpetually of one of the suns- the north pole could be the equivalent of the hells, for instance- filled with demonic servants of the evil sun. While the southern cap could be closest to the weaker sun and act as a sort of celestial paradise on earth. I'm not sure if it would be physically possible to align the planet's axis in such a way, but in a fantasy world this too could be chalked up to "magic".

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