cosmology
PC will be arriving from all campaign settings.
the world the PCs are walking into is very earth like. there is no native magic creatures. there is a trait that the plane has that saps energies from all matter. there are ways to overcome the magic sapping but doing so requires practice and experience.
the surface is enormous.
those that have a grasp of physics,
the plane is a ring cross-section of a Dyson sphere 96.92 million miles in radius. the ring is 440,000 miles across and is 500 miles thick. the ring is orbiting around a star at a equivalent of 1 earth gravity. due to the interior curve of the plane the edge is 250 miles higher than the sea level there is the earth typical air density decrease as altitude increases
there is an enormous "ocean" river that is bordered by 20,000 miles of forest and prairie land on each side of the Great River Sea. bordering the forested lands, extending to the edge of the plane, is 200,000 miles of sporadic vegetation on bare rock.