Time to begin the search for the hidden underground laboratory. It rests underneath Notre Dame. The trick is knowing where to go to get in. There is an underground entrance at the Southeastern end of Norte Dame. Head into the graveyard in that area and look for this opening to the underground:
Take the ladder down. Now follow the tunnel to the first divide in it. Creep up on either side of the divide. There are a few hostile monks just past where the tunnel rejoins itself. You can either fight through them or lure them toward you and use the divider to throw them off your trail. Use whatever tactic works best for you.
Continue down the tunnel ahead of you. The mechanism is just ahead. There are 2 more Hostile monks in front of the door. Lure them away with Cherry Bombs or just take them out.