It's one of the miscellaneous secondary weapons. There's a weapons-rack in there with a pink weapon shaped like a pelican. After you've beaten them, break the wall. Drop down when you reach that little spot with those ninjas and those transparent Blade Dancers.
Use the rail and continue to the wall to the right. Use the brick to get on top of the rail above. Activate the switch to get the big brick out. Once you have made it to the time-portal, don't go back to the past, but return to the center-aisle and the wall the Dahaka smashed. As you're wall-running through the center-aisle leading to the hallway to the time-portal (trust me, when you get there you won't have any other places to go, you'll know what I'm talking about), notice how the camera turns in front of you and shows the Dahaka get angry and smash a wall. As usual he will chase you all the way to a time-portal. You will be chased there by the Dahaka in the present. In the big room with the hanging gardens and the four levers that have to be activated in order to move on the first time you enter: later in the game you will find a way to the top floor.
