A grand ymir 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 Grand Ymir Chest in Evony TKR says:
Open the package to gain all the items in it.
Grand Ymir Chest is known to drop from a number of bosses or monsters upon killing, including Ymir.
Note: This list may be incomplete.
Grand Ymir Chest opens up to all of the following items: