The Ancient Sword Guard is the eighty-fifth treasure that can be found in Uncharted 2: Among Thieves.
The Ancient Sword Guard is found in Chapter 23 - Reunion. As soon as you work your way through the window, your eye will no doubt catch the location of yet another treasure, the fourth of nine found here. To acquire it, you must shoot it down, but you can't do so until you've worked your way to the rubble-strewn floor of this building. Once you're there, you can aim upward, shoot the treasure down, and then grab it once it's on the ground. Easy.