Shadowfell

The Shadowfell, also known as the Plane of Shadow, existed as sort of a counterpart to the Feywild, in the sense that it was a reflection, or "echo", of the Prime Material Plane. Unlike the Feywild, it was a bleak, desolate place full of decay and death.

Cosmology

The Plane of Shadow was considered only a demiplane. Demiplanes formed out of the proto-matter that ebbed and flowed about the Ethereal Plane, creating a finite plane with its own Border Ethereal whenever a critical mass was achieved. The largest of these was called the Demiplane of Shadow, and was made up of both positive and negative energy in equal measure. For those traveling through the Ethereal Plane, the "curtain of vaporous color" for the Demiplane of Shadow was the color silver. Wizards could shadow walk directly to the edge of the Shadowfell and travelers could use this plane as a transitive plane to traverse many Prime Material miles/kilometers very quickly. Clerics could use the plane shift spell to travel to this plane. If any were brave enough to cross the Demiplane of Shadow, it was possible to find the borders of other planes of reality.
Much like the Feywild, the Plane of Shadow appeared to echo the prime material. However, it shifted and changed as one moved along it, making it difficult to tell where one was. And, unlike the Feywild or the prime material, it touched some outer planes and allowed easy access to them, though it's unknown if these connections are permanent or where they are.

Shadow Spells

Known as "shadowstuff", the material of the Shadowfell could be manipulated by illusionists to form semi-real monsters and quasi-real evocations that were still effective (to a lesser degree) even if the target successfully disbelieved the illusion.
Another tool of illusionists was the shadow well spell, which could turn a creature's shadow into a temporary gateway into the Shadowfell. If successful, the victim was sucked into the Shadowfell through his or her shadow and spent a few moments in a pocket realm (a very tiny demiplane attached to the Shadowfell) being terrorized by dark phantasms. No physical harm resulted from this spell, but mental and emotional trauma resulting in a fear response was possible.
An unsuccessful attempt to build a gate to the Shadowfell could result in what was termed a failed Shadow Gate. A being that stepped through one was taken only momentarily to the Plane of Shadow, but in that moment a "seed of shadow" was implanted in their flesh. This could cause their flesh to be subsumed by shadow, and they would see it mist away into darkness. Many died, reduced to bare skeletons, but those who survived acquired the powers of a dark creature.

Alternative Name(s)

Plane of Shadow

Type

Plane of Existence