Investigate Inside the Sainte-Chapelle
When you gain full control over Arno, you’ll need to scale the front wall of Sainte-Chapelle and go inside. As soon as you go through the doors, turn around (as if you were going to leave again), then scale the wall up to the balcony. There will be an option to interact and progress the story further by placing the medallion in the beam of light. Don't be like us and wander around aimlessly for five minutes.
After sitting through a short cut scene, you'll see an on-screen objective marker on the other side of the room. Jump across the chandeliers, then cross the room using the rope, making your way into the marker on the other side. Once you reach your destination, take a leap of faith into the hole in the floor.
Once you regain control, the on-screen objective will require you to follow Bellec. There isn't any combat here, so enjoy the conversation as you make your way through the tunnels.
Following another cut scene, you'll find yourself in a dream like state that requires you to free run through a makeshift obstacle course. Start out by taking another leap of faith, then navigate over the crumbling floor as you attempt to reach your father.
As you continue, you'll come to what we found to be the trickiest part, and that's sliding down the slope while avoiding the pits of fire. Pay very close attention to the on-screen instructions on how to slow your progress, then inch your way to the broken piece of floor on the right side of the ramp. When you're clear of that, get back to free running through the linear obstacle course.
During the final part of the memory, you'll find yourself in a large crowd. Feel free to walk forward along the path between them, using your Eagle Vision to spot your target standing in the middle of the room. To avoid letting this man spot you, make your way through the crowd to the left, then sneak up behind him and go for the kill. Doing so will bring about the end of the memory and the sequence.
Welcome to the Brotherhood.