How to start: Speak with Xenia after Sacred Vows quest.
Description: The woman gives you three new maps. She also sends you out to look for Conch Shell. The pictures above show which map you have to use and the location of that point on the world map.
Jump down into the Underwater Cavern. A few enemies are inside of this location – their levels are scaled with yours. Reach the back of the camp and loot the chest. The item is inside of it. You can now go back to Xenia.