The Tibetan Ritual Blade is the fifty-fifth treasure that can be found in Uncharted 2: Among Thieves.
The Tibetan Ritual Blade is found in Chapter 17 - Mountaneering. The first of six treasures here can be found as soon as you descend the ladder after crossing the chasm with your guide's rope. Instead of going through the tunnel in pursuit of your guide, turn around and head in the other direction. You'll quickly run into a dead end, but you'll see the glimmer of a treasure along the edge of the dead end pathway that you can easily grab before continuing.