The Shadowfell, sometimes simply called Shadow, was a parallel plane from which necrotic energies and shadow magic stemmed. It existed as sort of a counterpart to the Feywild in the sense that it was a reflection or “echo” of the Prime Material Plane except that it was a bleak, desolate place full of decay and death.
Much like the Feywild, the shadowfell is similar to the Prime. The Prime world is reflected in the shadowfell as a grim illusion. Showing everything as a dying, dead, or sick vestige of the Prime world.
Arcane magic run just a freely here as in the feywild. Shadow and necromantic magic is stronger here than anywhere else.
Little is know about the Shadowfell, even less than is know about the feywild.
The Shadowfell’s reflection of the underdark has never been seen and there is no solid proof that it even exists.