Hundreds of years ago, the weak red sun finally flickered out. Every remaining settlement maintains its own little sun-star, which provides daylight to an area a few miles in diameter.
Every year, a ritual must be performed to renew each town’s sun-star. The ritual uses up gold. Adventurers are sent into abandoned, dark, horrifying cities and ancient dungeons, and told, “Return with life-giving gold!”.
If adventurers become rich enough, they can generate a new sun-star and start their own Point of Light settlement.
In such a world, normal plants and animals are at a premium, while metal can be foraged from the dead cities, so metal armor might be cheaper than leather. That makes this cold world the reverse of the Dark Sun setting.
The few orchards would be carefully tended, and no one would waste valuable wood on torches. Perhaps everyone knows a ritual that causes a gold coin to burn like a torch.
In the vast cold darks, the monsters are touched with shadow or aberration. The forests are hostile, and the shadows wait for the sun-stars to die.