From the announcement:
Fantasy Flight Games is pleased to announce The Trollfens, an expansion for Descent: Journeys in the Dark Second Edition that introduces new heroes, monsters, classes, quests, and more! Over the course of its five included quests (playable as side quests to a larger campaign, or as their own mini-campaign), heroes can discover and investigate secret rooms and rumors while the overlord spreads infection and brings a powerful new lieutenant into play: the massive troll Bol’Goreth.