A senior hydra 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 Senior Hydra Chest in Evony TKR says:
Open the package to gain all the items in it.
Senior Hydra Chest is known to drop from a number of bosses or monsters upon killing, including Senior Hydra.
Note: This list may be incomplete.