Shadowfell

Overview

The Shadowfell is a plane of shadow, death, curses, and lost things. The landscape is drab and lacks color and light; no sun, moon, and no stars adorn the vault of the inky black sky, and all things look as if the color has leached out, leaving nothing but black and white. There is constant rot in decay in the remnants of forests and swamps. These morose qualities reflect in the denizens of the Shadowfell; they were often somber or outright miserable with little creativity and passion. Still, a few choose seem to prefer living in this macabre place, turning the greying lands of shadow into their home. Many of these creatures were once fey that has been corrupted by shadow magic. The Shadowfell was a part of the Feywild before being corrupted and nearly pulled apart when the Raven Queen attempted to ascend. As a result, it shares close ties to the material plane like the Feywild and can be reached without magic by finding a portal in the material plane. Such portals are often tied to locations where powerful tragedies occurred or excessive necromancy was practiced. Unlike some other planes, the Shadowfell is not infinite and though it is very large, many other planes and places can be reached by walking beyond its borders. Along these borders, the shadows shift constantly making the very landscape change shape. In some cases, a part will break off unto itself, becoming a Domain of Dread.

Inhabitants

There are two deities that vie for control of the Shadowfell and lay claim to its lands: the Raven Queen and Myrkul, Lord of Bones. The Raven Queen was a powerful arch-fey that attempted to ascend to Godhood. She had a large following of Elves under her command as well as many powerful wizards. In her ascension ritual, her cabal of 13 wizards plotted to hijack it for themselves and split the power. The attempted corruption cause the ritual to spiral out of control and the Raven Queen cursed the cabal before fracturing under the cataclysmic failure of her attempted godhood. The Raven Queen, her followers, the cursed cabal, and all of her lands were pulled into a dark place of shadow, set apart from everything as its own plane. The Raven Queen's mind was splintered from the ritual, though it has partially succeeded and left her with a great deal of power. Her essence resides in the Fortress of Memories. Her followers, the Shadar-Kai, enforce her cryptic will on the lands of the Shadowfell and treat them as their own. 
Myrkul, Lord of Bones, was another immensely powerful wizard that attempted to attain godhood. In retaliation, he was cursed with undeath and banished to the Shadowfell by the gods after he failed to conquer the world of Oerth with his undead hordes. Weakened and terribly bitter about the situation, he empowers undead creatures throughout the Shadowfell from his Citadel of Bones. Most of all, he hates those who cheat death through various means of resurrection and sends his minions to the material plane to hunt those people and murder them, binding their souls through undeath into his service. Some lost souls are drawn to him within the Shadowfell and pledge their allegiance willingly to carry out Myrkul's mission of murder and death. These creatures and the undead are in constant war with the Shadar-kai, though neither side ever seems to gain an advantage.
Other notable inhabitants include the cursed cabal of 13 wizards who now resemble hunched bird-like creatures called Nagpa. They all reside within the Shadowfell, hiding from the Raven Queen and plotting their revenge. Rarely, a dragon of the pure shadow is spotted terrorizing the lands. Some unfortunate beasts may wander through the portals of the Shadowfell and become corrupted to form wholly new monstrosities. Roaming the lands are cloaked humanoids belonging to the Brotherhood of Mist, a secretive monastic order that seeks freedom from the physical form, and the Vistani, a wandering group of traders and mystic practitioners that frequent the Domains of Dread.