- Three air assassinations
- Two Phantom Blades kills
It’s time to hunt down Le Roi des Thunes in the sewers below Paris, but first you’ll want to complete the Opportunity Events that will help make your life easier.
As soon as the memory starts you can air assassinate the guard below and kill his friends, should they spot you and interfere. You can now walk over to the first chimney vent puffing out smoke and cover it with a piece of wood before climbing up the building, crossing the roof, and air assassinating the guard on the other side. The second chimney is nearby and you can cover it without hassle since the remaining guards have their back to you.
Equip your Smoke Bombs and then climb the buildings nearby and head toward the white circle with an (!) to spot a bunch of guards attacking civilians. You’ll want to save as many of the rebels as you can, so toss a Smoke Bomb into the group and then get that final air assassination to reach the ground and finish off the remaining guards.
The rebels will run off and attack the guards to the north, allowing you to walk by without having to deal with a fight and enter the alley. There are two guards here that you can double assassinate, if you unlocked the skill, and then you can jump the wall and sneak into the sewers via the mausoleum. Downstairs there are three guards that you should be able to quietly take out when the patrolling guard begins to walk away from the entrance and then enter the sewers.
Follow the tunnels until you reach a room with two guards and wait for the patrolling guard to walk left before you walk in and take out the resting guard, following by the patrol. In the next room you’ll be above the enemies, where you can air assassinate the stationary guard when the patrol walks away and then kill the patrol.
The following room has three guards that are spread out enough that you can take them out pretty easily by killing the one straight ahead and then the other two who have their backs turned. To continue, climb up the wooded up tunnel exit and enter the main chamber for a scene.
After the scene you can exit out the opposite tunnel and take the left split to avoid the first set of enemies. Wait for the three guards in the next room to split up and then air assassinate one when the rest have their backs turned or just jump in for a fight.
As you progress and slide under the bars La Touche will run away, but there’s no need to worry about him now and you can turn right and climb the ladder up to the main chamber.
Despite the assassination waymarker being in the middle of the room, a quick Eagle Vision will reveal that the situation is actually a trap. Ignore the main entrance and turn right to slide under the sewer grates and climb the ladder on the far side of the room.
Le Roi des Thunes is on the opposite side of the area, but you don’t want to try and attack him from here. Instead, turn right and jump down to kill the sniper next to you and then run along the beams to a locked door that you can pick. This tunnel will lead behind Le Roi des Thunes, so take your time and pull out your Phantom Blade and get a nice clean headshot before you kill him with a hidden blade for a scene.
Before escaping, make sure you get another Phantom Blade kill to complete the second optional objective. From the platform, hop down to the lower platform on your right and then the hanging platform in the middle of the room and into the tunnels via the waymarker.
If you blocked both chimneys the tunnels will be covered in smoke, which allows you to run through pretty easy provided you give the guards a wide berth, and you can complete the mission.