The super mutant tribes that plague Mount Desert Island are a problem for the settlers whenever they come into contact. Especially when the Sole Survivor reestablishes human outposts in the Fog.
This is one of several quests that take place at the various settlements the player can own. If inhabited and this quest comes up, the player will be asked to eliminate the leader of a band of trappers at a random dungeon. It's the Far Harbor equivalent of Stop the Raiding.