The Droknar Key is a unique reward trophy that each character may obtain once from Droknar in the Epilogue after completing A Time for Heroes. Exchange it with one of the Chests (listed below) located in the Epilogue for a unique item.
Please ask on the live chat for the stock level of this item. These keys can be very difficult to get hold of because to get a single key requires beating the campaign from start to finish on 1 character, which makes it rather limited since once every player has already done this and keys are used up, you would need more players to do it over and over, which in general doesn't happen that much.
Wait times for Droknars Keys can be exceptionally long.