The Ritual Crown is the seventy-fifth treasure that can be found in Uncharted 2: Among Thieves.
The Ritual Crown is found in Chapter 22 - The Monastery. There are a couple of treasures to grab once all of the enemies and their endless reinforcements are downed. The first can be found closer to your starting point, once you've breached the monastery walls. Specifically, you're going to want to enter the small building directly in front of and to the right of the door through the wall (if your back is facing said door). Once you enter the room, head to the open area along the huge chasm adjacent to the monastery, and look rightward. You should see a horizontal red wooden beam sticking out from underneath a stone platform. At the very end of the wood is a gleaming treasure for you to acquire. You'll have to get over to the wooden beam using the pieces of red wood along the adjacent wall, and then shimmy to the edge of the beam to grab the treasure.