"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.
You receive a vision of a rooftop "somewhere close by", where a rare moss grows; in the vision, you know the moss is suitable for greatly enhancing the effects of certain types of potions. Now go and find that rooftop. Don't fall off. (Of course, rooftop runners *will* be taken for miscreants by the local City Watch....)