Story related, cannot be missed.
This trophy is called Turning the Tide and is awarded after completing a certain portion of the main storyline. See below for details, but be warned as spoilers follow:
The quest name you complete to net this trophy is Enemies in High Places. After you are tasked with finding Fomorous Hughes within the lands of Detyre, you will eventually head through a series of quests that have you finding Hughes, meeting up with him, and discovering the true nature of Hughes' benefactor (Templar Octienne's) research. If you listen closely enough you will also glean a few tidbits of information about your character's (the Fateless One) past before the start of the game. Finally you will have to fight Templar Octienne (a real pain in the butt fight as well as the little bugger uses strong magic attacks and is constantly moving around). Once you beat him, (not kill him sadly) you will meet with another Templar and after you finish talking, the trophy should unlock.