Kalraka Dzeik is a sentient lightning storm with a following of cultists. It has much in common with a natural disaster or a plague; it starts at a single point but spreads like wildfire if given the chance. It is capable of terrible destruction, and will take heroic effort to stop, if it can be stopped at all.
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.