A royal thief chest is an item in Evony TKR (The King's Return). It can be found after killing certain monsters or bosses, with some of them listed below.
The description of Royal Thief Chest in Evony TKR says:
Open the package to gain all the items in it.
Royal Thief Chest is known to drop from a number of bosses or monsters upon killing, including Royal Thief.
Note: This list may be incomplete.
Royal Thief Chest opens up to all of the following items: