Well, not dying out would be a common interest. *Grumps* Yes, it wouldn't affect the humans in this case, and it would have been a group effort on one side or the other that made the terrible thing happen, but it should never be confirmed which side did it. I don't suppose that the simple fact that if the war continued they would most likely have killed each other off wouldn't work for you as a reason to stop fighting? It's always made sense to me, but maybe I'm being stupid and idealistic. It's hard to come up with reasons on a grand scale when you remove higher powers and gods from the picture my dear :P

As for that last little part, I've always thought it's more fun to hide stuff from humans when they themselves have NO relation to that which is being hidden, so for me, no magic as far as the human s are concerned. Makes fro creative explanations as to why some of the more public things that happen :P