Shadowfell

The Shadowfell overlaps the Material Plane much the same way as the Feywild. It is a dark mirror of the Material Plane. Vampires, werewolves, and all sorts of unsavory creatures make their home here. Portals here are known as shadow crossings and often form in gloomy places and manifest only in darkness.

Creatures have disadvantage on saving throws against fear and any effect that inflicts the fear condition has its duration doubled.

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.

Shadowfell Despair
When a creature suffers a loss or is otherwise saddened or stressed while on the Shadowfell, they must make a DC 10 Wisdom save or be affected by Shadowfell despair. 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.