Aythea
My D&D world.
Magic exists in hot spots, cold spots, and average spots. In hot spots, magic is more powerful and spells tend to do the maximum of what they can. In cold spots, magic is dampened and tends to do the minimum of what it can, or not work at all. The majority of the world is average, and magic works normally most of the time. These spots only affect arcane magic, such as that cast by wizards. Divine magic is unaffected, however there may be spots in the world where a certain deities magic is dampened or amplified for one reason or another.