The Darkness Within
This realm has three parallel Planes: The Ethereal, the Material, and the Shadow Planes.
The Ethereal and Shadow Planes are superimposed onto the Material plane, and all three co-exist and have slight influences on each other. Those powerful in magic arts can sometimes get a sense of what is existing in another plane at similar point if he focuses enough…
In the Ethereal Plane of existence, the typical “extraplanar” critters make their home. Here can be found daemons, devils, solars, divas, etc. waging their eternal wars on each other. Just as in the Material Plane, there are locations that are more likely to be controlled by one faction than another, so a savvy planar traveler will make sure that his presence will be at least tolerated before he pops unexpectedly into the Ethereal Plane.
In the Shadow Planes reside the souls of the dead. Most of the time, they are peaceful and restful, and waiting for the end of times. Sometimes, however, they are malevolent and seek to re-manifest themselves onto the material plane as undead. Sometimes, these spirits manage to accomplish their aspirations…
Because this world has three parallel Planes, the magics that it takes to get to one are not dissimilar to those that takes to get to another. Ergo, any spell that would transport a person into the Ethereal Plane may also transport them to the Shadow Plane. It is up to the caster to decide at the time of casting where he will go.