He hands it to his Ninja. Von Croy uses the Amulet of Horus to open a door. Beyond the door lie long dead Knights of the Crusades with red eyes. Find the entrance to the next room with the central round pillar and another rope. Looks like you should be able to do something in the alcove with the rope.
Nothing works, so keep exploring and find the stairs on the south side of the room.
Push the lever to open the large gold doors downstairs. Look around in the alcoves with the burning torches and find a Torch on the floor. Light the torch by standing near the burning torch on the wall and using the Action key. Go back downstairs to the room with the central round pillar and rope ledge. Do a simple jump into the alcove with the rope and light the rope on fire.