The Shadowfell

The Shadowfell is one of the two planes parallel to the Prime Material. It is a dangerous realm of death, decay, and darkness.
Its main inhabitants don't have classification like Fey. They are darkweavers, shades, shadar-kai, and other such shadow creatures. At best, they are twisted echoes of what exists on the Material Plane. It is a trait that even the landscape shares, appearing as the grimmest future of the Material that one could possibly imagine.
The plane dampens anything within its planar borders; emotions, magic, light, and heat are all subject to the rules of the Shadowfell. Hence why a spell like Light would only be partially effective, its distance halved in the best of conditions. Not even one sun, star, or moon makes even a pinprick of light in any sky within the plane... However, there is an exception to the rule, that being the art of Shadow Magic. Spells such as Darkness and Shadow Blade are enhanced, being almost--if not more than--twice as effective.
Some areas within the Shadowfell will tug at the lifeforce of any that wander through, noticeably draining their vitality, emotion, and willpower. Regardless of location, however, it can easily be said that nothing is pleasant there. Food will ooze dark blood; water is thick and smells foul; air is pervasive and cold. Time is not an exception, either, to the perverse nature of the plane. Time moves quickly in the Shadowfell, often forcing someone to endure years at a time when only a few days have passed on the Prime Material Plane. And years spent there often wear on a mortal body like decades would to a mortal body elsewhere.
Trying to exit the Shadowfell, unfortunately, is also very difficult. The longer that one spends there, the less will they have to leave. It is quite the manifestation of a downward spiral.

Type

Dimensional plane