For those who could be interested, this item, along with the Prominent Adventurer's Necklace, was given to us by a legionnaire of Seru.
A recuso leader had stolen important documents from Lord Inquisitor Seru.
Along with a few people we hunted down the recuso only to find a little key and a map to the Echo Caverns.
We headed to Echo Caverns to look for a chest or whatever.
We found a locked box that we brought back to the legionnaire.
This item was the reward from the Seru Inquisition after solving that lunar blackmail (was the name of the event).