This achievement is similar to killing all members in the Cult of Kosomos from the base game.
After meeting Darius for the first time, his son/daughter (depending if you choose Alexios or Kassandra the gender of Darius' child will be the opposite gender of you character) will ask you to meet them at their hideout to proceed through the DLC. During the cutscenes they'll describe who they are and who is hunting you.
A clue to one member is found iniside the hideout to which you can unlock Phratagoune the Keeper she can be found at the Bear Den in north Makedonia (I recommend that you kill all the bears near her for another member) after killing her you are unlock a clue to another member; Konon the Fighter who can be found at Xerxes' Sacrifice of the Nine.
One clue is inside the Olynthos Fortress near the nation chest which leads to Akantha the Deceiver she is found at the leader's house with the nearest Viewpoint being Asklepion.
Two members are found by the completing main missions in the DLC; Bubares the Conspirator and Timosa the Physician. For Timosa you are given the choice killing the doctor or the magistrate towards the end of the mission if you kill the magistrate she can be found (at least for me) at the seaside hideout which is close to border of Malis.
The final member is Echion the Watcher to which you must complete the side mission Dead Men Rises; you must go to a military camp and locate a clue inside a animal cage, then you must collect and give a blacksmith five belts and answer a code choose the answer 'and we are Order' and he can be found nearby Asklepion.
The final member is the Huntsman you face him towards the end of the DLC after beating him followed by several cut scenes the achievement will pop