In order to get this achievement you must be playing as the Dark Urge origin. You can find it in the origins in the create character menu, next to the companions.
After long resting a few times in the Shadow-Cursed Lands, you will be commanded by Fel to murder Isobel, the Selunite cleric keeping Last Light alive, I had already saved her at this point but still was able to do it. This will trigger the same fight that failing to save her does, so be prepared for a hard fight. Should you refuse to kill Isobel for long enough Fel will return and demand you kill your closest companion. You can do either of these, but you have to do one of them. Upon finishing, long rest again and Fel will reappear and the achievement will pop when you wake up from the long rest.
To my knowledge no actions you make in chapter 1 can lock you out of this but refusing to kill the targets will.