This thick, tough grass grows in deep forests of the Feywild or in areas where the Feywild’s magic filters into the material world.
Effect: A creature that falls prone in a square of grab grass must make a Strength check to stand up (use a DC based on the creature’s level, and usually an easy or moderate check).
Usage: Grab grass works best in an outdoor encounter. Use controllers and other creatures with the ability to move opponents and knock them prone. Then when the characters are prone and having a difficult time standing up, bring in the brutes to deal heavy damage. If you want to make it more difficult, choose brutes with reach attacks so that most characters won’t be able to attack from prone.
Published in Dungeon Master's Guide, page(s) 68.