I was pretty impressed with how
Final Fantasy: Crystal Chronicles used this idea. A dark poison mist covers much of the world, and a precious crystal keeps it at bay. The bigger the crystal, the bigger the radius of protection, so the big cities and towns all have big crystals at their center. But the light of the crystal fades gradually with time, and it needs a precious drop of liquid from a rare tree (the myrrh tree) to renew it. These trees are very rare, and grow only in highly dangerous areas, protected by hideous monsters and evil beings of all stripes. Their myrrh can only be harvested once every few years, as well. The job of the adventurers is to go to these places, make it to the myrrh tree, and collect a drop, and return home to renew the crystal's protection. To do this, they need to go into places thick with dark mist, so they carry a small shard of the crystal with them in a vase, which protects a small area around them, ensuring the party isn't affected by the poison mist.
It's surprisingly dark for a game with such childlike characters, but then, that game was surprisingly dark in a lot of ways.