Shadowfell

The Plane of Shadow (or Shadow Plane) is a transitive plane that coexists and, to some extent, overlaps with the Material Plane. It is similar to the Material Plane, with similar terrain and landmarks, but lacks colour and light. Despite this, plants, animals and humanoids live there. The plane changes constantly, meaning that no maps exist. Those that are drawn are outdated almost immediately.

Traits
Gravity and time are normal on the Plane of Shadow, and it is of infinite size. Magic functions normally, except for spells that emit light or fire, which are less predictable and prone to failure, and shadow spells, which are enhanced. Air is normal on the Plane of Shadow, but water is thicker and ichorous. Non-native residents can remain for an extended period without ill effects, but during the course of years, changes might occur. Emotions within individuals still exist on the Plane of Shadow, but they are less potent. The terrain is similar but not identical to that of the Material Plane, so if one were to transition between the Material Plane and the Plane of Shadow in a mountain range, one would arrive at the other side in a mountain range, although not necessarily the same mountains.

The Shadow Plane is not totally dark and it has a permanent level of brightness similar to a moonless night. Any sources of light, even magical ones, appear dimmer on the Shadow Plane than they would on the Material Plane. Fires also burn cooler than they would normally on the Material Plane, and the Shadow Plane is cooler.

Dark mirages
A dark mirage is a mirror image on the Shadow Plane of a feature on the Material Plane and is caused by the close proximity of the planes causing a "bleeding" effect. If one transitions from the Material Plane to the Shadow Plane and begins at a well-known location like a town, it is quite likely that the town exists on the Shadow Plane, but in ruins, or altered somehow. It is also possible that shadowy versions of its inhabitants exist on the Shadow Plane. This can be quite a distressing and alarming experience for anyone not accustomed to it.

Darklands
Darklands are areas of the Plane of Shadow that are extremely dark and bleak, and filled with negative energy. Living creatures entering an area of darklands have their energy sucked from them until they turn to ash and crumble. Because magic works (almost) normally on the Plane of Shadow, it is possible to shield against these effects with magic items or spells. The location of darklands often corresponds to graveyards, sites of battles or areas of potent necromancy on the Material Plane.

Shadow quakes
Although the terrain on the Plane of Shadow drifts slowly over the course of days, it is prone to sudden earthquake-like events called shadow quakes. These are extremely localized (within about a hundred feet (30m)) and their effects are similar to an earthquake on the Material Plane.

Travel
It is possible to access the Plane of Shadow through portals or via the shadow walk spell. Intermittent portals called vortices exist between the Plane of Shadow and the Material plane, and the entry and exit points are unpredictable, as is their duration. They open onto dark areas of the Material Plane, but by contrast, these appear bright on the Plane of Shadow, and the Material Plane seen through one of these vortices appears to have colours inverted (black is white and white is black). The Plane of Shadow connects with more than one Material Plane, making it possible for inhabitants of one Material Plane to use it to transition to another.

Inhabitants
The best-known inhabitant of the Plane of Shadow is the shadow. Others are generally evil and dangerous, including the dusk beast, the ecalypse, the nightshade, the shadow mastiff and the umbral banyan.

Shadow magic
Shadow magic uses the Shadow Weave as its source, taking the shadow energy and manifesting it elsewhere, as with the darkness spell. Notably, negative energy is not tied to the Shadow Weave, and spells or effects drawing on negative energy do not harness it.

Shadowdancers
Shadowdancers are those who use innate ability rather than arcane magic to harness the power of the Shadow Weave.

Shadowcasters
Shadowcasters are magic users who concentrate on studying shadow magic. This involves a significantly different path of study than learning most arcane magic, and requires a dedicated mind. Its secrets are closely guarded, so book learning is often not possible—shadowcasters are often chosen very selectively and then taught everything they know on the subject.

History
There is an unconfirmed legend that the Plane of Shadow began as a demiplane, but an order who venerated the plane's unknown creator built a fortress called the Shining Citadel and used the life, light and colour of the plane as its power source.