Here’s the location of the Unity At A Price lore object, which is required for the Rumbles of Thunder achievement.
First, you need to go to the Isle of Thunder, and get to the Hall of the Grand Imperion, which is located here:
Once you’re in the Hall of the Grand Imperion, you stay on the ground level. The lore object is on the rear wall from the entrance, so you’ll need to clear a couple of the Shan’ze Soulrippers to get to it. Here’s the picture of the Unity At A Price lore object: