- Two kills from hiding spots
- Don't trigger any alarms
In order to make navigating around the outside of Notre Dame easier, we're going to refer to your starting position as the bottom left corner, and the other corners of the building as top left, top right and bottom right. We may also mention the top, bottom, left and right. Just remember that we're always using your starting position as the point of reference.
We're going to assume you want to do this using stealth, the element of surprise, and in the most efficient way possible. For that, your first order of business will be to activate your Eagle Vision, spotting the thugs that stole the keys to Notre Dame. This will also update your objective.
Steal the Keys
The thugs who stole the keys are going to walk along the left wall of Notre Dame, heading toward the top left corner of the building. Stay on top of the roofs and follow, waiting for them to go in the bottom floor of an establishment. When they do, climb in through a third story window, then sneak down the stairs and pickpocket the keys from one of the thugs. You shouldn't get caught as long as you're very careful and sneak your way down.
In addition to killing Sivert, you also have the option to kill his associate, Duchesneau. His patrol route takes him along the right side of Notre Dame, after which he will start to patrol around the outside of the building. Now that you've stolen the keys, go back to the front of Notre Dame, but stay on the rooftops the whole time. You should see an exclamation point over Duchesneau's location. Move to the roof just beside him and activate your Eagle Vision, then immediately take a leap of faith into the cart full of hay near the gate that's being guarded by two soldiers.
Using your Eagle Vision will activate Duchesneau on his pre-determined path, which just so happens to take him directly to the cart full of hay you are hiding in. Wait for him to approach, then use your attack button to perform a stealth kill. Killing this man is an absolute must if you wish to follow our instructions to kill Sivert in the confessional.
It should be noted that the only real objective you need to complete is this one, so the few that we've helped you complete above are optional, although you must kill Duchesneau if you want to assassinate Sivert in the confessional. Theoretically, you could just walk in the front door and throw down with Sivert and his men, but this would almost certainly end in death and failure.
If you followed our tips to steal the keys from the thugs and assassinate Duchesneau, you will be close to where we're going to sneak you into Notre Dame. In fact, continue moving along the right side of the building until you're at the top right corner. Before you pass by the end of Notre Dame there is a building on your right with a sewer access point nearby. Climb down the ladder, then take a left when you find an opening. Go up a small set of stairs and press the button on the wall.
As soon as you enter Notre Dame you will activate the two challenges for this memory: two kills from hiding spots and don't trigger any alarms. Your first order of business, however, is to sneak past the two guards up ahead. You could also do what we did, which is use our Double Assassination skill to take them out simultaneously.
All this sneaking around is intended to set you up to kill Sivert in the confessional, but to do this you need to take out the two guards just a little further on ahead (not the two you just snuck past... or murdered). This is most easily done by letting them catch a glimpse of you, then using your recently discovered skills to perform cover kills when they move in to investigate. When they are down, look for a cart full of hay near the large crowd of people, then use your Eagle Vision to spot Sivert and activate his patrol. Stay in the cart and wait for his bodyguard to walk by. This will be your first of two kills from a hiding spot (The one on Duchesneau didn't count because the challenge wasn't activated).
Wait for Sivert to enter the confessional, then use your stealth mode to sneak into the right booth. This will trigger a cut scene, and when it's over, follow the on-screen instructions to kill your target, completing the unique kill. This will also complete the challenge to get two kills from hiding spots.
Escape the Area
With Sivert dead, exit the confessional and turn to your right. Approach and use the nearby lift, finding yourself on a balcony with a window to your right. Use the keys you stole from the thugs to unlock the window, then exit Notre Dame and make your escape. A quick look at your mini-map will show you the shaded area you need to leave. Doing so will complete the memory, as well as the sequence.