A new story is frequently being whispered in the dark corners of taverns across the country, telling of a traveler that occasionally appears in quiet, rural towns; that draws with him a plague worse than any other: a dragon's wrath.
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.