Rilutaan: The Western Marches
The Shadowfell, sometimes called the Shadow Plane or Plane of Shadow, is a dark reflection of the world. Many locations and geographical features of the world are roughly where one might expect to find them in the regular world, though in the Shadowfell they will have been changed by the dark nature of the plane. A mountain range may be made of harsh crags that seem like spears tipped at the sky. A forest may be a dark, dismal wood with gnarled roots that reach out to ensnare boots and tug on cloaks. The whole plan is suffused with a kind of eerie gloom. Sunlight does nothing here. At the best of times, the sky is a muddled, maudlin gray like on a deeply overcast day. More often, the sky is cast in utter darkness.
Undead creatures thrive in this place, as do other creatures that rely upon and hunt within the dark. It is not a place of light or happiness. Mortals that travel to the plane begin to suffer from prolonged exposure to its energies, growing more paranoid, apathetic, or outright mad.
The plan is also home to beings known as Dark Powers. These entities are utterly evil and are somehow trapped within the plane's confines. Their demesnes are demarcated by clear boundaries in impenetrable mists that block vision completely. Travelers can travel into the thick mists all too easily, but getting back out again is another matter entirely. The Dark Powers themselves do not seem to be capable of traversing the mists, so far as anyone knows.