Shadowfell

The Shadowfell is a plane in Dungeons & Dragons introduced in the 4th edition's World Axis cosmology and retained as part of 5th edition's cosmology. The Shadowfell is the plane of the dead, where the souls of all mortals that die travel and eventually make their way to the resident god of death - The Raven Queen, in 4e - to be sorted and sent on to their eventual afterlife, which lies "beyond the veil" (or in a deity's dominion in the Astral Sea) in 4th edition and back onto the Outer Planes in 5th edition. Unless of course the dead refuse to follow, which means that the place is crawling with the undead.

Later materials introduce the idea that it also contains a specific class of monsters found nowhere else. These beasts are essentially anthopormorphic - slightly - emotions, and congregate in places where the living may tread. They are accompanied by literal armies of Negative Energy-animated bone creatures, undead, and constructs, as depicted in Mordenkanen's Tome of Foes.

Like its brother-plane the Feywild, the Shadowfell still harbors mortal life despite its many problems. This isn't easy, especially since the plane casts a kind of mystical malaise that strengthens various dark feelings, such as gloom, despair, dread, paranoia, sadism and ennui. Shadowfell dwellers tend range from evil to insane to unusually (and often eerily) quirky - it says something when the Shadar-Kai, a race of vivaciously hedonistic, sadomasochistic, arrogant adrenaline jockies, are the consistently nicest race of natives.

Like the Feywild, the Shadowfell was created by the Primordials when they fashioned the World, this time as a dumping ground for that elemental proto-matter that was "too dark". Consequently, the Shadowfell mimics the physical appearance of the mortal world, but always... off in ways that symbolise death, or which are just plain creepy. A mountain range in the mortal world may, in the Shadowfell, be small, weathered and depressed looking, or taller and sharper, with viciously jagged rocks that cut the hands when touched, and in both cases will probably form weird outlines like screaming faces, fang-filled mouths or a rotting skull.

The Shadowfell is based on a combination of the Great Wheel's Plane of Shadow and the Negative Energy plane. Its Pathfinder counterpart is the Boneyard of Pharasma.

Fun fact: the Upside Down in the Netflix show Stranger Things is based off the Shadowfell.