The blackboard of the middle ages
A large steel shield in the form of a sea-shell attributed to Athena, the goddess of civilization, craft wisdom and war.
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.