encounter idea's

Haunted:  spirit's souls are stored in lanterns around a area that lights the area up.  Once the group enter the area they cannot leave until the lights are all extinguished.  The lights fuel the undead which revive after they are destroyed.
Library Lurker:  There is an intelect devourer hiding in a library and preying on the people who come to visit.  The group must find the creature and dispatch it before they become it's next meal.