A grasping bog is an area of deep, viscous mud that not only hampers movement but also clings to characters who tumble into it.
Effect: A grasping bog is difficult terrain. In addition, a creature that falls prone in a square of grasping bog is restrained (save ends).
Usage: Combine grasping bog terrain with monsters that can knock their enemies prone.
Published in Dungeon Master's Guide 2, page(s) 60.