(spoiler-light) Unmissable storyline achievement. You get this even if the Orlesians kick you out. After an attack on the Inquisition (and the "In your Heart shall Burn" achievement), You'll be able to do a mission in Orles concerning an assassination plot. Going there and completing the mission in any way (there's at least 4 ways to finish it) will earn this achievement.
While there, consider going for
Be warned that guides for that achievement are full of spoilers.