The Warchiefs are located on the Army menu standing on the wall. There are 2 of them. Hover over each of them and it will show you their bodyguards. Chose a warchief that has at least 2 bodyguards and then target one of the bodyguards. Take their health down until you can dominate them, and then Recruit them, on the next menu you can press square to turn them into a spy.
If the bodyguard is too high level to recruit, you will first have to shame them after dominating them and then target them again to recruit.
Once you have all their bodyguards as spies, you will get a new Quest called Betrayal. Starting this quest will unlock the trophy.
If you have to kill the bodyguard in your attempt to recruit them, you can send one of your recruits from the Army screen to Infiltrate the warchief by selecting Command on them and then highlighting the warchief and infiltrating. They will then get a quest that you can watch to fight in an arena. They have to win the fight for this to work, so it could take multiple tries depending on the level on your recruit and the target they have to fight. If they lose, they will die and you will have to get another recruit to try again.
Alternatively, you can go to one of the fast travel towers and advance time. This will change the army and put new bodyguards in place.