Plane of Shadow
The Plane of Shadow, also sometimes known as the Shadowlands, is a former demiplane that within merely the last few years has grown and expanded beyond the Ethereal Plane to become a full plane in its own right. Partially coexistent with the Prime Material Plane, it has bare few connections with either the Outer or Inner Planes; as a result, this is one place where a Prime sage is actually more likely to have a good breadth of knowledge than a Planar.
The origins of the Plane of Shadow are unknown; there is no record of its creation, with even the earliest recorded mentions referring to it as if it has always existed. Some theorize that it first budded from the Ethereal as a result of a happenstantial formation of two nearby vortices to the Positive and Negative Energy Planes, just near enough for their energies to blend as they flowed out, while just far enough that they didn't mutually annihilate one another. Exactly such vortices can be found near the "center" of the plane approximately 50 miles apart, though the center is so far from any settled regions that even locals are often unaware of their existence; however, it's unknown if the vortices formed the once-demiplane, or if the demiplane formed the vortices.
Others believe that it formed in the same process that first gave rise to the Prime itself, given its nature as a pseudo-mirror. In any case, it's likely that the former demiplane is near as old as any.
As mentioned, most all inanimate matter in the plane is a blend of positive and negative energy known as "shadowstuff", solid within the plane but rapidly transitory outside it. It is this substance that is drawn on by spells that reach to the Plane of Shadow, and it seems to have natural mimicry properties, allowing it to easily be formed into duplicates of various objects or phenomena, in shape if not appearance. Perhaps as a result, the entire plane appears as a monochrome version of the Prime. In those regions where the plane is coexistent with a portion of the Prime, it appears as a near perfect mirror with regard to topography and terrain. A forest in the Prime would overlap a similar shadowstuff forest in this plane, a sea in the Prime manifests as a sea of faux-water in the Shadowlands, and so on, though not tree for tree or rock for rock; far more generally than that. Relatively minor changes in the Prime — such as the construction of a small building — are usually not mirrored. Major changes — such as the construction of a city or an explosive volcanic eruption — aren't always immediately mirrored (though if as a result of purely natural processes, they more than likely will be), but the Plane of Shadow will eventually shift to again mirror its double. Even individuals can be mirrored upon the Plane of Shadow via natives in their form and appearance (though most likely not their personality); this occurs most often in areas of heavy population such as major Prime cities. Interestingly, it appears that the Plane of Shadow may be expanding towards the Outer and Inner Planes, as some recent reports describe regions around the rare links to either to have the same mirrored nature.
On this plane, as the name suggests, there is little to no light; not much of a problem for natives, but outsiders should be wary. The entire plane seems draped in an ever-present fog that almost seems to sap light — this "fog" is actually small particles of shadowstuff suspended in the faux-air, but while it does obscure vision, it's the nature of the plane itself that dampens light. No region of the plane is ever brighter than that of twilight, and even outside light sources work poorly; natural sources never illuminate more brightly than a torch, while spells that bring forth light illuminate only a quarter as far as usual, and for only half the time. Further, the plane is devoid entirely of color of any sort. While it does not drain color as the Wastes, here color simply cannot exist; instead, all on the plane, even that not native, appears in stark black and white without even shades between. This effect lasts only while on the plane, though natives and shadowstuff still tend to be very muted in appearance, as if always shaded with black tones regardless of the light about them. Even heat is muted on the plane. While fire still burns just as well, across the entire landscape there's a chill that no campfire or hearth can seem to relieve.
Far from being equally blended throughout, the balance between positive and negative energy can instead vary from place to place throughout this plane. The common reports of, for example, a city on the Prime being mirrored as an abandoned, crumbling ruin in the Shadowlands most often occurs in areas where negative has slightly more prominence than positive, while those places where, again for example, a peaceful forest upon the Prime is mirrored as a deadly overgrown jungle are areas where positive has more prominence than negative. In some areas, this imbalance can tilt even farther. Such places are known by locals as "Brightlands" or "Darklands" respectively, each appearing just as their name implies, and both can be deadly even to those native to the plane. These areas tend to form most often in places where the Prime itself holds a great saturation of either energy, as the energy leaks across to Shadow and tilts the scale; thus, as it is far more common that negative energy will saturate a region than positive, Darklands are far more commonly encountered than Brightlands.
Connection with the Prime
Because it is coexistent and coterminous with the Prime, many spells and rituals have been developed over the centuries to exploit it. There are many known spells that pull shadowstuff from the plane to mimic creatures or effects; the spell shadow walk allows one to straddle the Plane of Shadow and the Prime to speed travel greatly; and of course the shadowdancer uses an innate connection with the Plane to perform many of her feats. However, there are some regions where the Prime overlaps with the Plane even more greatly. In some it's merely a thinning of the veil, as the greater presence of the Plane leads to thicker shadow, dimmer light, and a general feeling of unease amongst those unused to it. But occasionally, the veil tears entirely, and in these places transit between the two planes can be far more easily accomplished. Much as elemental vortices, they spawn natural gateways between the two, allowing one to step from one to the other without effort, for so long as the connection holds. Indeed, many on the Prime have mistakenly been trapped in the Shadowlands, and vice versa, thanks to the transitory nature of these rifts.