A horde of devious, ferocious, tenacious, and atrocious creatures, beasts, and monstrosities to populate the northern realms of your fantasy. Get them while they're cold!
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.