Shadowfell

Gloom, despair, hopelessness, and loss suffice the Shadowfell and weigh down upon all those within it. The Shadowfell is an echo of the Prime Material Plane, much like the Feywild and Weave, within its dark reflection of Runetalras, filled with darkness, death, and an unnerving sense of being watched. Forests hold sinister shadows, mountains loom taller and more ominous, and everywhere the natural order seems twisted and bent towards a darkly malevolent end.
The nature of the Shadowfell draws evil and undead creatures to it, and this attraction has led many planar scholars to link the realm to the Negative Energy Plane. There certainly seems to be some truth to this connection, and it’s worth noting that the Feywild – another echo plane, but one filled with vibrant and often violent life – seems just as strongly linked to the Positive Energy Plane. Are these two sides of the Material Plane the result of the Positive and Negative Energy Planes? Or are the Positive and Negative Energy Planes a result of the Shadowfell and Feywild? Debates continue in scholarly circles
From a more conventional standpoint, the Shadowfell is a place where fell creatures dwell, and its location on the other “side” of the Prime Material Plane makes its interference much more common than other decidedly evil outer planes, such as the Abyss or the Nine Hells. A pervasive gloom in a forlorn crypt, the shadow cast by a tombstone in the moonlight, or the yawning darkness beneath a creaking gallows all can contain portals to the Plane of Shadow, allowing monstrous creatures to slip between the cracks and wreak havoc.
The Shadowfell is also the home to the Domains of Dread, unique and isolated realms locked away from the rest of the plane by insidious Dark Powers. These unknown macabre masters of darkness draw powerful men and women away from the Material Plane at their darkest moment, trapping them and their lands. Barovia, home of the vampire lord Strahd von Zarovich, is the most famous, but other domains exist, scattered across the Plane of Shadow.
Monsters of all types stalk the gloomy wilderness of the Shadowfell, and between them havens of civilization are hard to come by. Most larger cities have dark reflections within the Shadowfell, of these cities Ashgloom and the Queen’s Nightmare are notable examples. While other famous sites include the Twisted Forest, Palace of Memories, and the Midnight Towers.

Portals to the Plane

Spontaneous portals to the Shadowfell from the Prime Material Plane are not uncommon. These portals appear places of darkness and shadow, like dungeons, deep cave systems or mines, dark forests, and more commonly the Underdark. There are semi-permanent portals which only open under specific conditions usually on specific dates, the location of the moons, or times of night. Permanent portals to the Shadowfell only exist in the darkest corners of Runetalras where the sun never shines, such as the Underdark.
Certain spells in older tomes and spellbooks are known to pierce the veil between the Prime Material Plane and Shadowfell as well, but the secrets of these have been lost to most of the current arcane world.

Powers of the Plane

The Endless Darkness, Lord of Shadows - Primordial of the Shadowfell
The Primordials are entities of raw power that are believed to embody the very planes that exist upon, their powers are said to rival even that of deities. The Primordials are said to be the first entities born of their planes, ageless in nature and bottomless in power. The Endless Darkness is the embodiment of shadow and negative energy. There is no doubt that the Endless Darkness is the most powerful entity upon the Plane.
Dark Powers
The Dark Powers are unfathomable, hidden forces manipulate life, death, and reality within Shadowfell. These mysterious, deathless beings are the architects of the Domains of Dread, and have secreted these pockets of terror deep within the Plane of Shadow. The Dark Powers watch and influence events within their dominion.
The Dark Powers don't move in the shadows of the Domains of Dread; they are the shadows. They're the Mists and the darkness. They're the reason water manifests as rivers and stars appear in skies divorced from universes. They are the wardens that hold the Darklords prisoner. Ever-present and inscrutable, the Dark Powers are akin to deities within the Shadowfell, but they are almost unknown to mortals and desire no worship.
The Raven Queen
The deity known as the Raven Queen is the most active upon the Shadowfell, with the bastion of her followers being found within the Palace of Memories. Several temples run by cultists lay scattered about the plane, and she has an ongoing battle with the Demon Lord Orcus, Prince of the Undead. The two have sparred through proxies countless times across the multiverse, but the Shadowfell is where their most fierce showdowns have taken place.

Denizens of Shadow

There are few native creatures to the Shadowfell, those being the various kinds of undead, shades, and shadows. A number of other creatures have made the Shadowfell home including fiends, aberrations, and other humanoids that have become trapped like the shadar-kai.