"" is a hidden quest in that you can complete for many rare rewards by discovering a secret cache of loot. The treasure you unlock contains Gold, important resources, and new gear to make your character stronger.
However, to obtain this quest's bounty, you'll have to solve a mysterious riddle regarding four famous warriors. Similar to unlocking secret areas of the Lighthouse, you have to use specific companion abilities to solve parts of a puzzle in this quest.
At the very least, you need Lucanis' Spite skill to open up new paths within Sharksmouth Mountain. Leveling up the bonds with individual companions can help you in the long run by giving them more abilities to assist you inside and outside battle.
Before you even know how to find Sharksmouth Mountain, you have to advance the personal side quests of the character Taash.
This will eventually allow you to start by traveling to the Rivain Coast area. Once the mission is unlocked, your map will mark this location as a point of interest for your party to visit.