It is the toxic plane of darkness and power. It is the hidden place that hates the light. It is the frontier of worlds unknown.
The Plane of Shadow is a darkly lighted dimension that is both coterminous to and coexistent with the Material Plane. The Plane of Shadow is a world of black and white, an ever-changing landscape of stark contrast. The sky is always black and the land a powdery white, with no shade or hue anywhere; color itself has been bleached from the environment. It is otherwise appears similar (but not exactly identical) to the Material Plane. The sky above, for example, is always a black vault with neither sun nor stars. Landmarks from the Material Plane are recognizable on the Plane of Shadow, but they are twisted, warped things - diminished reflections of what can be found on the Material Plane. Despite the lack of light sources, various plants, animals, and humanoids call the Plane of Shadow home.
Parts of the Plane of Shadow continually flow onto other planes. As a result, precise maps are next to useless, despite the presence of landmarks. If a traveler visits a mountain range during one use of a shadow walk spell, the mountain range may still be there the next time, but the individual mountains may have moved about. Precision is a lost cause in the Plane of Shadow.
The terrain of the Plane of Shadow is usually similar to the area where the traveler enters from the Material Plane. If a wizard travels onto the Plane of Shadow from a forest, she first sees an equivalent shadow forest. If she starts underwater, she appears within a shadowy sea that behaves like a Material Plane ocean, so her water breathing spell still works. However, as she moves away from her entry point, the terrain of the Plane of Shadow changes dramatically, although it usually bears some resemblance to the corresponding terrain on the Material Plane.
The Plane of Shadow overlaps the Material Plane much as the Ethereal Plane does, so a planar traveler can use the Plane of Shadow to cover great distances quickly. The Plane of Shadow is also coterminous to other planes. With the right spell, you can use the Plane of Shadow to visit other realities.