"We just thought the winter was worse'n usual. Turns out we had an unexpected visitor. Wasn't until poor Dave and Glenda... died... that we finally got a clue. By then it was almost too late. You know how many of us'll never be the same? Oh, you doubt me, do you? Look in my eye, son, and tell me what you don't see. Yeah, I thought so. Damned Snow Devil!"
All you need to stop a brain is a bullet.
At regular intervals, a god takes control of your world's weather. Choose the gods at random each turn of the interval and begin applying their effects subtly. Their power will depend on the strength of those who worship them.
For civilised people, worship of a god actually wanes during that god's month in an effort to reduce their potential destructive effect.