there are two divine forces that make up the magic system: the primordial dark, or Prazninu, and the primordial light, or Clarus.
Prazninu is destruction made manifest. physically, it is an oily liquid that is completely black, and if it touches almost any physical matter, it slowly turns into more Prazninu. spells that use the primordial dark are only to destroy, because that is what it is. perfect annihilation.
Clarus is a more insidious evil. it does not have a physical form, only manifesting as a brilliant white light. Clarus corrupts sentient beings, making them follow in its ultimate goal of purifying the entire universe into nothing but light. spells using the primordial light can have a variety of different effects, mostly healing or utility-based.
these two forces have been at war since the universe began, so long that whatever sentient deity may have been manifest have dissolved into nothing but aimless light
the player character (name pending) travels this desolate, destroyed kingdom after a bloody battle between the two forces, and end up either picking a side, destroying the world in the process, or completely destroying both forces and ending magic forever
that's the basic idea