Genre: High fantasy/heroic fantasy
Genre: Mid fantasy
Genre: Low fantasy
Genre: Science fantasy
Genre: Steampunk
Genre: Science fiction
Genre: Post apocalyptic
Magic: High magic
Magic: Mid magic
Magic: Low magic
Magic: No magic
My elaboration on the "guilt" and "death" mechanics that I posted directly before Ghostman's post deal with that by eliminating the good/bad aspect altogether. The magic and demons become more powerful simply through the very natural process of death. And they can be removed from the world. But natural disaster, plague, famine, and war are all things that could bring them right back with a significant resurgence in strength that is outside their "normal" level of presence that has nothing to do with good and evil. It's related simply to the raw magical energy released when living creatures die. Larger, more magically inclined creatures would release more energy and power the crystals more, letting more demons through.
This could lead to any number of storylines that make the whole point of good and evil totally ambiguous. Perhaps a kingdom establishes a religion that deifies these demons as a natural part of existence and has blood sacrifices to deliberately let them into the world in small enough quantities that they can be managed. A neighboring kingdom may view this practice as evil, and misguidedly go to war to end this practice, oblivious to the fact that the death toll their war is causing is actually throwing an uncontrollable fuel on the fire. Who is the real evil then? The one trying to moderate the process, or the one making it worse by trying to stamp it out?