The Shadowfell

The Shadowfell, also known as the Land of Mournful Dead or the Spiritlands, is one of several planes of existence. It is home to deceased souls neither accepted into Sanctuary nor claimed by the Writhing Chaos. These forlorn, lost spirits dwell in a bleak, desolate place of decay and death.

Description

The Shadowfell exists as a grim mirror of the material plane; where a forest stands in the material plane, one can expect to find a similar forest in the shadowfell. Landmarks in the shadowfell are typically recognizable, but altered in dark ways--a castle may be made of bone rather than stone, a forest stands dead and rotting, etc.
Shadow magic and necromancy draw their power from this plane of existence, as consequence shadow spells (e.g. Shadow Blade) and necromancy magic are naturally enhanced in this realm of darkness. Conversely, magic that produces light, fire or healing are prone to failure, and light (natural and magical) is only half as effective.
The Shadowfell invokes an unnatural apathy in travellers misfortunate enough to find themselves here. While time is more or less the same as in the material plane, the bleak reality of the Shadowfell and the absence of day cycles makes hours feel like days, and days like weeks.

Shadow Crossings

While most prefer to avoid this land of mourning and gloom, those who wish to travel here and lack powerful magic (e.g. Plane Shift) retain options for arriving (or swiftly un-arriving) from the Land of Mourning Dead. Shadow crossings exist throughout the Shadowfell--places where the darkness is so complete that one can pass between the realms, using a shadow as a gateway. Shadow crossings exist in places of darkness and death (e.g. graveyards), and may only be traversed from the material plane at night (shadow crossings in the Shadowfell are permanently active).

Shadowstuff

Shadowstuff is a black, semi-solid substance native to the Shadowfell. It can be manipulated by illusionists to conjure unholy creatures, weaponry and other items. Items made from Shadowstuff are hazy and impermanent in the material plane, dispersing to nothingness within hours of being exposed to sunlight. Weapons made of Shadowstuff are capable of harming ghosts, specters and other incorporeal beings.

Inhabitants

Though the Shadowfell boasts relatively few inhabitants (and virtually none of them friendly), there are native beings that call this grim land home. Among the most sentient and willing to negotiate, or even offer refuge, are the occasional necromancer here to hone their craft, and the Shadar-kai--a shadow-touched lineage of elves. Among the less diplomatic beings are undead, shadowfiends and Aspects of Death, all of which are capable of sensing the presence of the living and hunt them relentlessly within the Land of Mourning Dead.