Go to the assassin's tomb and find the large water-filled room before the room where you get one of the seals required for the Armor of Altair. This room has an obstacle course. Activate the lever before going through the course and stand on the edge of the floor, then look at the water. Remain idle until a large shape swims across the water. Use the lever again after the timer resets and stand on at same location. After a moment, a giant tentacle will appear.