While this Campaign is, in part, a playtest for D&D Next, and therefore attempts to provide access to most of the assumptions presented in that material, it is also a thematically-colored Campaign in its own right.
Themes which the Campaign, through setting details & rules adjustments, will endeavor to maintain or emphasize include:
The ever-increasing decline of civilization into a Dark Age of lost technology & lost hope.
The diminished availability of resources, due to the loss of elven & dwarven technology, goods, & materials, and due to the loss of natural resources due to the tainting of woodlands & fertile land.
The bursting of the dark humanoids onto the surface world.
The cut-off & few-and-far-between nature of human bastions, due both to the turning of intervening lands into wasteland, and to the capture of intervening territories by the humanoids.
The distinct nature of the directions taken by each major settlement center in their quest to maintain viability in this new realm.
Regions of particular ill-health in the landscape (leafless woods, steaming green sludge rivers, etc.)