Well you get this simply by beating the game. Below I will put a walkthrough of the last mission on the Hard difficulty. so if you don't want to see the ending before beating it yourself don't watch. This achievement is called "A Personal Contract"
Credit of the video goes to the youtube user "iNTiMiDaTeGaming"
There is 3 parts of the last mission Absolution. Cemetery Entrance where you have to make it to the entrance. Burnwood Family Tomb where you locate and kill Jade. Finally The Crematorium where you have to kill the Praetorians and kill Travis.