Every so often, while roaming some of the bigger cities, you will see someone free running, trying to get away from you. For this achievement, you must stop the courier, either by tackling him, or killing him. Try not to mix them up with thieves, or corrupt noblemen who you can kill to decrease your notoriety. Couriers can be identified by the red capes they wear, and are marked by the red on your map. Once you find one, they will be marked on your map, so it makes it much easier to find them. They aren't as fast as you are when it comes to running, but if you are having trouble catching up to one, use throwing knives. As long as they don't find a way to conceal themselves, you should usually be able to kill them outright. If they get far enough away from you though, they will disappear on your map, and you will have to wait for them to come again.