The Bleak Night Fog is a task from Una that you can do every day to get valuable rewards!
To access this, you must open Una's task menu using Alt + J, and select from the daily quests.
The quest requires you to talk with the crew members to listen to the stories of ghosts at sea. Then you need to check the ghost ship story.
To perform this task, you must enter in a ghost ship event that you can do once a week. The event appears several times a week, but it is not available all the time. To know the time of the next event, you can use the compass icon under your minimap to know the next occurrence of the event.
It is interesting to do this quest for the daily rewards that allow you to upgrade your equipment, but also in the long term to increase the reputation specific to this quest to go up to level 3 and get the item Ghost Ship: Eibern's Wound.