The Shadowfell is a dark and shadowy realm where colors are dark and faded and shapes are obscured at best. It somewhat pararells the mortal world. Creatures from this realm have shadow origins. The Raven Queen's home is built within the Shadowfell.
The Shadowfell runs parallel to the World and the Feywild, and access to this dark realm can be reached by both planes where they connect. Unlike the Feywild, the Shadowfell copies towns, castles, roads and all other features, but only as a twisted and decaying fashion. 
The creatures of the Shadowfell appear dull and darken to mend into the shadows that cover the plane. Although, despite being native to the world, some creatures even manage to live well in the infrequent settlements of the plane — the city of Gloomwrought, for instance.[HoS:8]
Emotions are equally dulled, turning to apathy and melancholy. High spirits are rare in the realm where the dead pass through on their final journey. Negative emotions—fear, anger, greed—might be amplified. Such feelings are easier to raise in the murky atmosphere, leading visitors from the world who linger down a self-destructive path.[HoS:8]
- Cosmology - Design & Development, Richard Baker