Way to unlock: Talk to Iron Bull in Skyhold while having high approval level and after completing "The Captain of the Chargers" quest.
The quest-giver: Iron Bull (M12,22)
Talk to Iron Bull (M12,22) in Skyhold. You must have a high approval level and play all previous cutscenes before you can start this quest. Then go with Iron Bull (as companion) to Storm Coast to start the quest.
After cutscene talk again to Iron Bull and go to the marked point on the hill. Eliminate all Venatori in two camps on your way. Don't forget to search the chests - in one you will find legendary helmet.
Then walk to the fire. Interact with it to give a signal for attack. During next conversation you must decide whether Bull warriors should retreat or try to hold the position. Holding the position is better option (you unlock the "Failed Assassination Attempt Inquiry" operation and get additional 400 influence points and more approval points from Iron Bull). It's the last thing you must do to complete the quest.
Rewards for completing the quest:
- Experience (lots)
- 400 influence points
- 3 power points
- Legendary weapon