You can get the throne accessories by going to the war table and doing ANY of the gather resources missions with any of the agents. I recommend not doing the gather coin. Most of the others like gather herbs or gather leather, etc. are much faster and only take 10-15 minutes while gather coin takes 45-50 minutes. Each time you do a mission you might get a random throne accessory. After I got my second throne accessory of the same type, the achievement unlocked.