Here's an interesting concept: gods. Most often, this shows up in fantasy gaming, where clerics have divine powers (often including the ability to cast certain magic spells) granted by their deities.
Most games don't give much thought to how exactly this works. Do these gods exist? If so, do they all exist, or do certain pantheons exist while others are simply the imagination of their followers? If more than one pantheon exists, how do they interact?
I think that the most elegant analysis of this conundrum is the cosmology created by Rich Burlew (ok, heavily borrowed by Rich Burlew from many already existing sources) for
The Order of the Stick. If you read his entire
oeuvre, you eventually learn that "in the beginning," so to speak, there were four pantheons of gods who created the world. Their inability to co-operate resulted in the slaying of the Gods of the West (based on real-world Greek gods; Zeus, Hera, Athena, Ares, &c.). The remaining pantheons (North, based on Norse mythology; Odin, Thor, Loki, &c. -- East, based on Babylonian mythology; Marduk, Isthar, Tiamat, &c. -- and South; based on Chinese mythology; the "twelve gods" include Dragon, Rat, Pig, Monkey, &c.) thus agreed to stay in their respective areas and not interfere directly in the regions of the other groups. This is why clerics are important; they can be the agent of their gods in other places. Later, the goblins and elves developed gods of their own too, who were grudgingly welcomed into the celestial realms.