Hello, adventurers of Blue Sea and Black Sails! Below is a detailed step-by-step guide on how to complete the Brutal Secret Mission.
1. First, open the map to find "Cruel Secret Mission", press the space bar to track the mission, and then go to the question mark on the map to mark the mission point.
2. After getting close, press the F key to go ashore, and you will find a new stronghold [Sunken Gold Mine]. After talking to the merchant, you can accept the "Cruel Revenge Commission". (You might as well buy your blueprint from the vendor)
3. Open your cargo bay, find the spy's clue, and select View Treasure Map so it will appear in the upper right corner.
4. From the treasure map, you can learn that the treasure is buried next to a stone with a crab symbol.
5. Following this clue, you can get the "Hidden Relic Letter" on the table inside, and then go northwest.
6. Immediately afterwards, you can find a stone with a crab symbol behind the pirate bonfire. When you get close, a red light will appear.
7. Finally, press the F key to dig for items, and you will find the "Message Before Death". Then, the "Cruel Secret Mission" is completed.
Following this step, you should be able to successfully complete the Brutal Secret Mission. Good luck!