Cleansing the Land DLC, Far Harbor Quest
This quest will be triggered or can be aquired during Best Left Forgotten quest.
Related points of interest
Stages of "Cleansing the Land" quest
1. Locate the Nuclear Launch Key
Seems DiMA uncovered the location of the launch key for the submarine in the Nucleus. I'll have to get into the safe room in the Harbor Grand Hotel. Luckily, I've got the code - 485130.
2. Listen to Safe Room Security Tape
Appears the Harbor Grand Hotel safe room had an old recording system. Maybe it can point me to the launch key.
3. Obtain the Nuclear Launch Key
The launch key is hidden on a boat at Fringe Cove docks. Safe combination is 44, 6, 75, 12.
4. Use the Launch Key to Destroy the Nucleus
I've found the launch key that could destroy the Nucleus. If I use it, it could change the future of the whole island.