Shadowfell

The Shadowfell, also called the Plane of Shadow, is a dimension of black, gray, and white where most other color has been leached from everything. It is a place of darkness that hates the light, where the sky is a black vault with neither sun nor stars. The Shadowfell overlaps the Material Plane in much the same way as the Feywild. Aside from the colorless landscape, it appears similar to the Material Plane.
Landmarks from the Material Plane are recognizable on the Shadowfell, but they are twisted and warped—distorted reflections of what exists on the Material Plane. Where a mountain stands on the Material Plane, the corresponding feature on the Shadowfell is a jagged rock outcropping with a resemblance to a skull, a heap of rubble, or perhaps the crumbling ruin of a once-great castle. A forest on the Shadowfell is dark and twisted, its branches reaching out to snare travelers' cloaks, and its roots coiling and buckling to trip those who pass by.
Shadow dragons and undead creatures haunt this bleak plane, as do other creatures that thrive in the gloom, including cloakers and darkmantles.

Shadow Crossings

Similar to fey crossings, shadow crossings are locations where the veil between the Material Plane and the Shadowfell is so thin that creatures can walk from one plane to the other. A blot of shadow in the corner of a dusty crypt might be a shadow crossing, as might an open grave. Shadow crossings form in gloomy places where spirits or the stench of death lingers, such as battlefields, graveyards, and tombs. They manifest only in darkness, closing as soon as they feel light's kiss.

Domains of Dread

In remote corners of the Shadowfell, it is easy to reach horrific demiplanes ruled over by accursed beings of terrible evil. The best known of these is the valley of Barovia, overlooked by the towering spires of Castle Ravenloft and ruled by Count Strahd von Zarovich, the first vampire. Beings of the Shadowfell called the Dark Powers created these domains as prisons these "darklords," and through cruelty or carelessness trapped innocent mortals in these domains as well.

Optional Rule: Shadowfell Despair

A melancholic atmosphere pervades the Shadowfell. Extended forays to this plane can afflict characters with despair, as reflected in this optional rule.
When you deem it appropriate, though usually not more than once per day, you can require a character not native to the Shadowfell to make a DC 10 Wisdom saving throw. On a failure, the character is affected by despair. Roll a d6 to determine the effects, using the Shadowfell Despair table. You can substitute different despair effects of your own creation.

Shadowfell Despair

d6	Effect
1-3	Apathy. The character has disadvantage on death saving throws and on Dexterity checks for initiative, and gains the following flaw: "I don't believe I can make a difference to anyone or anything."
4-5	Dread. The character has disadvantage on all saving throws and gains the following flaw: "I am convinced that this place is going to kill me."
6	Madness. The character has disadvantage on ability checks and saving throws that use Intelligence, Wisdom, or Charisma, and gains the following flaw: "I can't tell what's real anymore."

If a character is already suffering a despair effect and fails the saving throw, the new despair effect replaces the old one. After finishing a long rest, a character can attempt to overcome the despair with a DC 15 Wisdom saving throw. (The DC is higher because it's harder to shake off despair once it has taken hold.) On a successful save, the despair effect ends for that character.
A calm emotions spell removes despair, as does any spell or other magical effect that removes a curse.

Evernight

The city of Neverwinter in the world of the Forgotten Realms has a dark reflection on the Shadowfell: the city of Evernight. Evernight is a city of cracked stone edifices and homes of rotten wood. Its roads are made mostly of trampled grave dust, and its few cobbled streets are missing enough stones that they appear pockmarked. The sky is corpse gray, and the breeze blows cold and humid, bringing a chill to the skin.
The city's living residents include mad necromancers, corrupt purveyors of human flesh, worshipers of evil deities, and others who are able to make themselves useful and crazy enough to want to live here. But the living are a minority in Evernight, for the bulk of the population consists of the shambling dead. Zombies, wights, vampires, and other undead make the city their home, all under the watchful eyes of the ruling caste: intelligent, flesh-eating ghouls.
Rumors abound that this foul place mirrors one city on every world.

Type

Dimensional plane