You are playing as Connor's Father, Haytham Kenway Start the mission by entering Theatre Royal to find yourself in the lobby. You will give your Invitation to a guy at the door, and continue without giving him your coat.
You will hear someone call "Ladies and gentlemen, you are requested to kindly find your seats". Find your seat by following the Green Circle Icon. Other guests in this party will also be heading towards the seats, so you can follow them.
Locate the glowing target. You now need to find a hidden area to climb. Get up from your seat. Go left and use ladder. Then slide right on the wall. Climb a level higher when you reach a pole, slide further right, and climb another pole. Make sure the people inside the theatre boxes don't see you. Enter a small room with three doors in it. You now need to find a way backstage.
Go towards the green circle until you reach a locked door, and follow the screen prompts to Pick the Lock
Now you are in the Backstage area. Jump to the sun and clouds prop from the edge of the wooden plank, then climb a ladder, and then jump to the crescent moon prop, keep working your way left, and climb the platform on the other side.
Kill the man in the private box and take his pendant (This amulet is the the key to some storehouse. It is the key you were looking for). A little boy witnesses the murder, he is too shocked to say anything so ignore him and you now need to exit the opera house.
Find a secluded area to climb (Try ladder to your left)
Birch and Kenway Dialog
Birch: "Evening Haytham"
Birch: "I can't tell you how happy I was to hear they'd mounted this revival. Gay's best work by far. Have you seen it before?"
Kenway: "Once. My father brought me here as a child, though I remember little of it. And I don't suppose tonight will afford me the luxury of a proper viewing either."
Reginald; "No, I' afraid it won't. On to business, do you see him?"
Kenway: "He's seated in one of the boxes above"
Reginald: "The stairs are watched. You'll need to find another way up."
Kenway: "I already have"