This is the Ancient Arrowhead Treasure; You must find all 20 dreamcatchers to reveal this location and be able to collect the Ancient Arrowhead.
The Ancient Arrowhead is a unique item that makes Arthur's stamina last twice as long when using the bow.
The treasure is hidden in a cave behind the water fall at the Elysian Pool. Once you are inside the cave, follow the gold water up the tunnel. Do not fall in the crevace on the right side.
When you get to a large open room, go half way up the right side. You will see some cave paintings. Part way in on the right side you will see a small hole in the wall. You can inspect this to collect the Ancient Arrowhead.