As others have stated this is done on mission 9 of the DLC. You need to fight and disarm the governor and use the brand he is using as a weapon to kill him.
I screwed this up a few times initially, so maybe this will help some people. The governor always takes the same path to the same place and you don't have to follow him, you just need to get there without being more than about 60 meters or so away from the governor. I took an alternate path through the center of the town and so I wasn't getting tackled by his guards every two seconds. I ran to his destination and had time to reload the gun and shoot several of the guards there to thin them out. I killed the rest using smoke bombs and hidden blades, then switched to fist, countered (B), disarmed (A), smoke bombed again and hit him with the poker to pop the achievement.