* SPOILERS *
Simply create a human, non-mage character and you will be the "Last of Your Line". Play through the storyline, and once Duncan recruits you into the Grey Wardens and you leave Highever, the achievement will pop.
One note, regarding a very nasty bug in this origin story: When you are in the main hall, after defeating Arl Rowe's men there (notable by having the only mage you face in this origin story), DO NOT talk to Ser Gilmore. Wait for him to talk to you.
If you talk to him first, it will prevent a scripted event from occuring. That event unlocks the side door to the main hall. If you bypass that event, it will make this achievement unobtainable without loading a previous save or restarting the origin story.
Otherwise, this is pretty straightforward, just go tell your brother to march to Ostagar, go to sleep, and then fight your way to your father, the Teryn, and you will be joined there by Duncan.