This quest begins automatically, right after you finish the first act and eventually become a imperial ruler.

You should assist Harrim by visiting the old stunt fortress with him in which the trolls made their lair. You’ll need this location in the primary story quest of this chapter.

When you arrive, then go inside and come down to the lower level. Inside this story quest, you need to defeat their leader and investigate the anvil that Harrim will break to end the quest.