Cleansing the Land

Cleansing the Land is a main quest and achievement/trophy in the Fallout 4 add-on Far Harbor.

Detailed walkthrough
Cleansing the Land is a quest that requires the Sole Survivor to destroy either Far Harbor by utilising the Wind Farm kill switch code, or by using the nuclear launch key and thus destroying the Children of Atom at the Nucleus. Although destroying one of the two factions finishes the quest and grants the achievement Cleansing the Land, the player can still choose to destroy both factions.

Both codes needed for the destruction are obtained in the previous quest Best Left Forgotten.

When choosing to destroy Far Harbor the player can optionally give the nuclear launch key to High Confessor Tektus. Do note that by doing this the player can't go back and destroy the Nucleus as well, should they desire so. Tektus will praise the player, but then urge them to destroy Far Harbor. To do so the player will have to visit the Wind Farm Maintenance and enter the kill switch code in the Turbine 003 regulator terminal. This shuts down the wind farm, killing off the electricity needed to run the fog condensors around Far Harbor. As a result, Far Harbor will be engulfed in the fog and the creatures that come with it. A message will be displayed at this point that you are now an enemy of Far Harbor. After activating the code and returning to Tektus, the player will receive Atom's Bulwark and the title 'Inquisitor', along with the perk Inquisitor of Atom as reward. Far Harbor itself will turn into ruins covered in fog and overrun by creatures, a legendary Fog crawler being amongst them. Almost all Harbormen are dead, with only a few making a desperate last stand.

When choosing to destroy the Nucleus the player will have to insert the nuclear launch key in the Vessel. Doing so will turn all Children of Atom hostile and will initiate a 30 second countdown upon which the submarine will go into lockdown, followed immediately by the launch of the nuclear warheads. As long as the key wasn't given to him, is also possible to convince Tektus through speech checks that it is the will of Atom to launch the nuke, which will allow the player to leave the Nucleus without being attacked. The player has 30 seconds to leave the Nucleus. Once outside the building the player will be safe from the explosion. Once the Nucleus explodes, the player will be given the perk Far Harbor Survivalist. Upon returning to Far Harbor there will be a commotion, with Harbormen gathering around and Captain Avery announcing that there was indeed a nuclear detonation and that all Children of Atom are dead. Allen Lee will react happily about the news, but Avery will insist upon a moment of silence.

When talking to Allen he will call the player a hero and reward them with 1600 caps.

Bugs

 * Choosing to use the launch key and then staying in the Nucleus to watch the launch may crash the game.
 * Confronting DiMA about the wind turbines or the nuclear launch codes might not be possible, despite him being an active quest marker. Maybe it is caused by Reformation already in progress.