There are two troll stacks and also a darkling army stack. You have to kill all of them to progress the quest. Look for a big troll in the mountains to the north, the darkling army nearby, and a troll until by a cave to the northeast (all this is assuming "up" is north).

There are a few parts in the campaign that aren't always crystal clear. If you get stuck, try to remember the general task, then see if there's anything strongly related. In particular, think about "finding more" and "going around" if quests get you down in campaign mode.