This quest begins immediately after The Reeve of Wincestre. Now that you’ve identified Reeve Selwyn as a member of the Order of Ancients known as The Gallows, it’s time to eliminate him.
To begin, you’ll need to go to the market in Wincestre, which can be found in the northwest of the city. Once you reach it, a cutscene will begin during which Selwyn speaks of executing two citizens of Wincestre. After the scene, you’ll need to assassinate Selwyn by any means that you prefer. You should be able to get an easy headshot on him from the position where you spawn, killing him in one hit. Otherwise, you can approach from behind the gallows and assassinate him in a more traditional way.
If you want to create a bit more of a chaotic scene, you can release the two boar from the pens beside where you spawn after the cutscene. Run away and let them into the guards, who will be distracted by their attack as you focus on Selwyn. However you do it, once Selwyn is dead you will need to finish off the rest of the guards, which include regular soldier units and one Banneret. When you’ve defeated all of them, confirm the kill on Selwyn to begin a cutscene, after which the quest will be complete.