"The Wizard of Waterdeep" objective should be Claim the Crown of Karsus. Once you descend underneath the city and confront the netherbrain, proceed with the story as usual. When the emperor says he wants to absorb Orpheus, let him. Continue proceeding until you get to the netherbrain boss battle. Once defeated, you should be presented with a dialogue option to betray the emperor. Do so, and watch the cinematic until you earn the achievement.