Plot Hooks

   As the parties travels, they come across a populated city with its key feature being a colossal obsidian obelisk. It is located in the center of the city acting like a sundial. Not only does this denote the passage of time to the townsfolk, but when the shadow cast by the obelisk falls upon  the city; portions of the city get transported to the Shadowfell. For a short duration of time the affected portion is displaced and is returned once the shadow passes. Anyone who ventures into a "shadowed" part of the town, only walks through shadowy forms of the existing areas.  

    *Through a ritual of arcane  illusionary magic placed upon the obelisk by a wizard               wanting to learn more about the negative energy plane, mysteriously the wizard has yet   to be seen or heard from in quite a longtime. Now because of this the city folk have tried in the past to remove the obelisk but a few sections of the city where permanently banished in shadow during their attempts. Now the city guard is vigorously posted around the obelisk and patrols are suspiciously frequent to prevent further mayhem.

 Because of this many occupants have either moved away or dealt with the daily occurrence, scared people just know to lock their doors and hideaway for the duration, But more courageous folk venture out into the Shadowfell during these times to look for  trade, adventure and exotic oddities. There is even a healthy thieves guild presence that regularly use the planar transition to smuggle goods.

 *To loop this hook into your game, make the party stop in the city for supplies or a story plot point. Also be sure to have the party arrive when the main gates are not obstructed by shadow, possibly midday.