Start down the stairs into the tunnel that appears in front of Arno. When you reach the bottom of the stiars you will see a lot of painting swirling around you. At the bottom of the stairs perform a Leap of Faith off the edge of the platform. This will bring you to the next part of the area. Arno will stagger up from the ground after a long fall.
Start moving forward toward the door that you see far in front of you. Keep running ward the door. You will need to free run across a number of obstacles. Just keep going forward. Things are closing on the end when you have a bit of floor drop from underneath you. Just climb up the platforms from there and then slide down the slope. Watch out for the fire spouts that erupt from the ground. Just keep running forward after that. You will see a shadowed hooded figure kill Arno's father. There is a quick cutscene that ends with Arno holding the faithful letter from some to Francois De La Serre.
When play resumes, you will need to start heading toward the building. This time you need to do more platforming than running. Follow the shadowy figure toward the house. The ground will collapse and reveal a number of platforms. Head slightly to the right. You will see a platform just out of reach. Approach it and it will drop down to your level. Jump onto it. Turn left on it and climb up the part of the platform which rises up. Turn to the right at the top of that. Jump over to the next platform over there. Run and have Arno jump toward the left-hand platform ahead of you. Wait for it to drop down. Turn and jump to the right after that. Continue in that direction and climb on up the next few platforms ahead of you. Arno will lead from the end of the long platform over to some smaller ones. Turn to the left and follow these platforms as they lower you down some. Head forward onto the platform with the red carpet. Start running toward the doors and you will see the murder of De La Serre again. The screen wil go black and the scene will change.
You are surrounded by shadowy figures now and need to approach and kill the one that has stuck down Arno's Father and De La Serre. Head forward into the crowd in front of you. Get close to the front of the crowd but keep a little ways in from the front. You will see the figure but you cannot let this figure see you or they will strike you down in an instant. Instead, circle around behind the Assassin from either side and kill them that way. You should use Eagle Vision to track your target through the crowd. Once behind the Assassin just move in for the kill.