The Shadowfell (ʃædoʊfɛl)

The Shadowfell exists as a sort of counterpart to the Feywild, in the sense that it is a reflection, or "echo", of the Prime Material Plane. Unlike the Feywild, it is a bleak, desolate place full of decay and death. There is no Sun within the Plane and, while many myths have spread attempting to explain this, they all contradict each other, providing no clear answer.

It is the toxic plane of darkness and power.
It is the hidden place that hates the light.
It is the frontier of worlds unknown.

The Shadowfell is a deceptively small Plane, covering only about a quarter of the total surface area of Tellus. With this, a strange, but related, phenomenon occurs in the Shadowfell. Time moves four times slower within it than it does on the Prime Material Plane. One day on Tellus proper is the equivalent of most of a week in the Shadowfell.   Due to this, one would expect technology to advance, relative to Tellus, at a fast pace. This is held back, however, by the incredibly harsh environments and lack of resources. Most of the technologies used in the Shadowfell are of Demon-make, imported from the Nine Hells in exchange for the only resource the Plane has to offer, Souls. Though the practice is frowned upon by most, it is still used even by the governors and rulers of Shadowfell Cities.