The Shadowfell Dimension

The Shadowfell is a dark forbidding dimension inhabited by many races, all now changed by this plane of existence. It's creation is tied to the Feywild, and and the Seelie & Unseelie Courts. When the Feywild came into existence the emotions & energy that created it was made up of life & light energy. Some creatures that exist here were evil and hated the Feywild & all it stood for. At the same time as the Feywild came into existence, the dark negative energy that was generated at the same time, began to coalesce into a mirror image of it. The evil creatures that had formed in the Feywild sensed this negative plane and the majority travelled to the Shadowfell. Their initial thoughts & outlook shaped the primal energy into a dark mirror of the Feywild. Gradually it took on aspects of that plane, and others as more creatures became aware of it's presence and travelled there.
When someone looks into the Shadowfell, they see a plane made of dark smoke, where the light and warmth of other worlds has been stripped away. Little colour remains, and blacks, greys, and darkened iron are the prime colours. The Shadowfell leeches the essence and vitality of any creature that travels here and warps them emotionally, taking away any spark of goodness, and replacing it with negative emotions. Many of these creatures also take on abilities that involve shadows and using the dark to it's full effect. Also, if a creature comes to it from another established world or plane, and they have the strength of will they can claim a portion of the plane for themselves, warping it into something similar to region of the world they came from.
Within the Shadowfell there are whole races found nowhere else, while others are the remnants of others who moved to here. The Raven Queen & the Shadar-Kai Elves are one such group that travelled here after being forced out of the Feywild. They have a large empire here, and the Raven Queen has close connections to another group that regularly meets here, the Unseelie Court. The latter is exceptionally powerful here and many Dark Fey live here, and use hidden portals to travel to the Material Plane, and the Feywild to cause havoc on both of these places.
One aspect of the Shadowfell is the link to other even less savoury Planes. The Hells & underworlds of many religions are linked to the Shadowfell with prominent & permanent portals that are found throughout, sometimes more than one. Each is rarely used though, as what awaits on either side is generally not welcome in the other, and as seen as hostile acts to cross over from one to another. The reason seems is that many creatures, like the Elves of the Feywild, look upon the Shadowfell as a kind of underworld or hell of their own world.
Saying this, there is a little known secret about the Shadowfell that only a few scholars & Occultists know. At it's initial creation, group of beings from another hell dimension seen the chance to embed themselves in this plane and help maintain it, as well as enforce their own will on it. The beings are stepped in myth and little is known about them if anything. They are known as the Dark Powers, and have helped shape the domains of many creatures here, as well as the minds of any creature that has settled here. One horrifying aspect of the Dark Powers is their ability to create demiplanes here using the energies of the Shadowfell. In doing so, they have created an infinite set of these known as the Demiplanes of Dread, of which to imprison some of the most evil beings in the Multiverse. The most powerful and oldest so far has been Strahd von Zarovich and his domain of Ravenloft. Nobody knows exactly why, though it seems that the Dark Powers feed of the energy of these demiplanes and act as guardians to the rest of the Multiverse by removing evil creatures that could change worlds for the worst.