A treasure box 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 Treasure Box in Evony TKR says:
Open to receive a random Treasure Fragment
Treasure Box is known to drop from a number of bosses or monsters upon killing, including Jormungandr, Typhon, Golem and Witch.
Note: This list may be incomplete.
Treasure Box opens up to potentially any of the following items: