Terra Cottas At Large is a side quest in the Orient (Act III), given by Wai Zhen, a deputy of the guard standing at the western entrance of the City of Chang'an. He will tell you of many Terra Cotta Soldiers wrecking havoc in the city. However, the guards won't attack them as they are superstitious of getting cursed when attacking or killing the clay soldiers.
You will have to find and defeat five groups of Terra Cotta Soldiers in order to complete the quest. They are identified by their purple names and usually found in areas behind those large wooden gates. When you have successfully cleared an area, some of the guards will move in and you can progress to the next area. Note: To the north-west you'll find a bridge leading to the Chang'an Palace - the Terra Cottas beyond the bridge are not part of this quest (however, another side quest is completed in the palace).
When you have defeated all Terra Cotta Soldiers you receive an update. Return to Wai Zhen to get the reward and to complete this quest.
Reward: 26000 Experience, random Magical Weapon