To find Assassin's Tombs, you must complete the mission "Novella's Secret". From there, you will gain the ability to find and visit all of the Assassin's Tombs. If you haven't filled your map out yet by synchronizing with the view points, the tombs will not show up on your map.
This tomb starts off with a chase, as you must follow a messenger and kill him. The end of the chase leads to a fight with all of the guards in the area. When this is done, it's time for some timed platforming. You must pull a lever, and then go around the area, grabbing onto other levers in order to unlock the main door. There are four levers in the area you must pull before the time limit is up.