Editing Skein of Worlds:The Transitive Planes (section)

== The Shadow Plane ==
If, while on the Astral Plane, you attempt to move away from everything, if you move in the right direction, somewhat inward, you might notice the dull gray light of the plane beginning to fade. Eventually you'll enter an area dominated by shadows. This is the Shadow Plane. One way of describing it is the Astral Plane's Ethereal Plane. This description may not be entirely inaccurate. While it's possible, if not easy, to reach the Ethereal Planes from the Astral Plane, it seems to be much easier from the Shadow Plane. Unfortunately, distances between places on the Shadow Plane seem to be somewhat more related to the distances on the material plane. 

A sphere on the Shadow Plane is not simply a giant portal like it would be on the Astral Plane. If you get close enough, there is an actual surface to land on. The shape of the surface seems to have little to do with the shape of the sphere it's connected to. On these shadow spheres, there are occasionally pools of... something. These pools are portals to the Ethereal Planes connected to the same sphere, but not always to just the first layer.

These shadow spheres are inhabited, but these creatures don't seem to have much to do with the creatures from the connected spheres.