From 1d4chan

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. According to 4e 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 - 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. The specifics of this path to afterlife might vary greatly depending on the setting, as the dead from FR are supposed to get to Kelemvor in his City of Judgement, while the dead from Eberron go to a their own version of Shadowfell to rot away with no chance to become a petitioner in the Great Wheel.

(If the souls go down into the Shadowfell when they die, do they drop into the material from Feywild when born? This fits nicely into the phylosophy of energy flowing downwards through the planes from positive to negative, but it was never developed in any capacity.)

Later materials introduce the idea that it also contains a specific class of monsters found nowhere else. These beasts are essentially vaguely anthropomorphic 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 Mordenkainen'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. A sidebar in Tasha's Cauldron of Everything says that the Vistani can also be found wandering around, finding dark secrets and helping out any well-meaning travelers unfortunate enough to got lost there.

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.

Shadowfell is supposed by some to be a metaphor for all things painfully lawful: stagnation, cyclicity and stillness, while at the same time reflecting the Prime Material in real time. Because of that, permanent settlements and landmarks that spend enough time in one place on the Prime will at some point gain their reflection in the Shadowfell. This is why twins of major cities can be found there, like Evernight or Gloomwrought.

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.

The Cosmology of the World Axis
Far Realm
Astral Sea
Feywild Prime Material Shadowfell
Elemental Chaos