Now that we know what Shadowfell is the adventure starts to sound much more interesting.
The Shadowfell is the land of the dead, where the spirits of the deceased linger for a time in a dark reflection of their previous lives before silently fading beyond all ken. Some undead creatures are born in the Shadowfell, and other undead are bound to it, but some living beings dwell in this benighted realm.
Like the Feywild, the Shadowfell also reflects the mortal world imperfectly. Towns, castles, roads, and other objects built by mortal kind exist in the Shadowfell about where they should be, but they are twisted, ruined caricatures.