The light is failing.
Not in a philosophical or metaphorical sense, not like good versus evil or hope versus despair, but in a most horrifying and terribly literal sense: The light is failing.
Each day, the light from the sun grows dimmer. Each time a lantern is lit, its light shines a little shorter. Shadows grow longer, nights grow colder. Even magical illumination seems to be losing its luminance. The world is marching inexorably towards a time of utter and complete darkness, and none can explain why.
Some are taking this in stride, namely the races possessed of greater vision than humankind (the elves are worried, the dwarves are scarcely concerned save for the growing lack of warmth, etc.), and the division of response is increasingly threatening to explode into a political division.
The party is conscripted by a very panicked council of philosophers to seek out the cause of this seeming apocalypse and, most importantly, discover if it can be reversed.