Find someplace suitable in the center of the Northridge Lumber Mill in Western Plaguelands to place the Barrel of Plagueland Termites.
Now that we have plenty of termites, take this steel reinforced barrel to the Northridge Lumber Mill south of Hearthglen--you'll find it to the west of the road that leads you up to the town.
Once there, place the barrel in the center of the lumber mill; someplace high, like on a crate or something.
After you open it, the rest should take care of itself. Just come back here and I'll reward you for helping me.