Once you are separated from your party, and step through the mirror, there will be a short "cut scene". Afterwards, Oengus will join your party. You will not get this achievement right now though! First you must get to the end of the dungeon, and right before you leave he will give you a choice to go through an optional door. Behind the door is a maze puzzle. You have to look at the statue's shield in the background, and then fly through the door with the same symbol right next to it. Once you get to the end of the maze and leave again, he will rejoin you and you will get the achievement.