For this achievement you must complete everything inside the Walls-of-the-Ruler fortress. the main objectives are: kill the commander, kill the two captains, and collect the six treasure. the earliest you can attempt this is during the main mission when you are tasked to assassinate the three targets at the start of the DLC. if you didn't complete all the requirements during that mission, you can return to the fortress and grab/kill what you still need.
*For those who are missing treasures, one is in underground passageway (use senu to find the entrance) as for the other five, I apologize but I can't pinpoint their exact location my only advice is to use senu as much as possible while inside the fortress you can tell when senu is close to a treasure is while in hover mode you'll see a white outline it means you close and by looking around (still in hover mode) you'll get the treasure icon pop up and try to mark the position using senu.