Invisible Fortress:Shadowfell pocket

Shadowfell pockets are small (usually) areas of Creation which have succumbed to the darkness of the Shadowfell.

Shadowfell pockets are the result of the Shadowfell intruding upon the land. Shadowfell pockets can be formed with powerful magic, or passively created when a large number of people die in the area over a small period of time. Anyone attempting to leave a Shadowfell pocket after sunset and before sunrise will find themselves in the Shadowfell itself, rather than the area of Creation outside the pocket they entered.

Major cities and roads, especially the city of Whitewall (whose entire construction was sanctified in the name of the Unconquered Sun) and the Great Northern Road (the primary route into Whitewall), have magical protections against the formation of Shadowfell pockets.