Fallout 4 features several quests that can play out differently based on the choices you make during it. These choices sometimes present themselves in siding with a certain faction over the other, as is the case with the Battle of Bunker Hill quest, and a similar situation unfolds when you get to the Cleansing the Land mission.
RELATED: Fallout 4: A House Divided WalkthroughThis is the finale quest in the Far Harbor DLC, requiring you to pick a side between the two main groups on the Island, the people of the Far Harbor Settlement (Harbormen) and the Children of Atom. Your decision will affect the future of the island as the side you ally with will become the primary residents of Far Harbor.
You can begin Cleansing the Land by obtaining the wind farm kill switch code and/or the nuclear launch key. Both of these can be acquired from the previous quest, Best Left Forgotten, which you also have to complete beforehand. The launch key can be in a motorboat west of Fringe Cove Docks. The kill switch code can be found under a mound of dirt near DiMA's cache, northwest of Rayburn Point.
Once you get the two items, it is up to you which of the two main factions you want to help. You can destroy both groups if you're feeling extra evil, but you probably shouldn't if you want to acquire some unique and fun items from the DLC. As previously stated, the mission and what your objectives are will be different depending on the side you help.
If you want to help the citizens of Far Harbor, make your way over to the Nucleus. You'll want to head to the Vessel specifically and insert the nuclear launch key into the missile status terminal inside the missile control room, which will launch a nuke heading straight towards your location.
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