The Trigram Thogchag is the sixty-sixth treasure that can be found in Uncharted 2: Among Thieves.
The Trigram Thogchag is found in Chapter 18 - Heart of Ice. This treasure is in a pretty obvious place, though you'll need to actually look in the proper direction to find it. The key is the gigantic, evil-looking statue that turns round when you get to the central pillar. Cross the statues that come out of the water and stand in front of the aforementioned evil-looking statue. Take aim with your gun (almost straight up) and look at its head, and in particular its crown, you'll see the gleaming of a treasure up there. Wield your P08-9mm and shoot the treasure down, and then walk forward to claim the treasure once it's fallen to the ground before the downward steps.