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.
This 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.
How To Begin Cleansing The Land
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.
Assisting The Harbormen
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. Doing this will make all the Children of Atom turn hostile so be sure to bring your best weapons, your best suit of power armor, and healing items if need be.
You could, if you pass a speech check, convince High Confessor Tektus that launching the nuke is the will of Atom, which will not turn anyone in the Nucleus hostile.
Regardless of your approach, you will have 30 seconds to evacuate and get outside. Otherwise, a lockdown will occur which will trap you inside and prevent escape. Once you're out of the place, you'll be safe and be able to witness the explosion. Now return to Far Harbor and talk to Allen Lee for your reward.
Assisting The Children Of Atom
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If you wish to help the radiation worshippers, head over to the Wind Farm Maintenance location which will be marked on your map if you haven't found it already. You'll also have the option to give Tektus the nuclear launch key, but it's not needed to complete the quest. You then will have to use the Turbine 003 regulator terminal and enter the wind farm kill switch code, that code being ) % T 3 M p e $ t {.
Doing this will kill off the electricity needed to power the fog condensers that keep the fog off of Far Harbor. Thus, the fog engulfing the rest of the Island will now enshroud the settlement and cause the creatures that come from it to destroy the town. Longfellow, a companion, will leave you and be permanently hostile towards you. After this, head back to the Vessel and talk to Tektus to receive your prize.
Rewards For Completing Cleansing The Land
Your rewards for finishing Cleansing The Land in Fallout 4 will vary based on whom you chose to assist in the quest.
- If you helped the Harbormen, you will get 1600 caps and the Far Harbor Survivalist perk. A perk that gives you +5 to all resistances, like damage, poison, energy, and radiation.
- If you helped the Children of Atom, the Atom's bulwark power armor piece will be given to you along with one of two perks (depending on your approach in The Heretic or Witch Hunt quests). Being aggressive in one of those quests will give you the Inquisitor of Atom perk and title, a perk that gives you a bonus to weapon damage. For every 100 rads you get, damage is increased by 10 points until you reach 900 rads. From there damage is increased by 100 percent. Taking a peaceful approach or ignoring those quests will net you the Crusader of Atom perk and title which does the same thing.