Yesterday I happened to be going by a players base as it was in the final stage of decaying. All of their thralls were still present and a bunch of loot bags were on the ground where the base has previously been. I checked the bags and found a legendary chest Ancient Key.
Today I went to Sepermeru to use the key on the Rhino Boss’ chest. For some odd reason the game kept telling me that I needed a key to open the chest.
Here are 2 screenshots showing the message that I needed a key and my inventory where you can clearly see that I indeed have a key. I have not tried the key out on any of the other legendary chests in the Exiled Lands.