One of my favorite atmospheric phenomenon is fog. Honestly, I just love the stuff. It's one reason I like winters in Georgia. There's generally a proliferation of cool, foggy mornings. Sure, spooky things can hide in the mist, particularly the walking undead. You always gotta watch out for that, particularly if you're a hero in Descent. In fact, the Mists of Bilehall expansion that was just released is all about undead in the mist.
Rumors of an undead army are spreading and it's up to the heroes of Descent to 1) find out if they are true and 2) do something about it if they are. This expansion is unique for Descent sets in that there are no new heroes to join in the battle. No, the bad guys get all the fun this time around. There's new monsters and new tiles, but only the heroes already available will be able to be used (which, obviously, means you need the Descent main game to play). Along with the box set, there's three Lieutenant Packs that are available to further expand the Overlord Player's options.
All of these kits are available now from your LGS or the Fantasy Flight webshop.