Trapper Attack

Trapper Attack on (Location Name) is a radiant quest in the Fallout 4 add-on Far Harbor.

Detailed walkthrough
While in a settlement in Far Harbor, a settler may approach the player character requesting for help against an incoming attack of a trapper party. Upon accepting to help, trapper attack groups will spawn around the settlement, and the player character must then hold off the attack.

Once all attackers are dead, return to the settler who requested help, who will tell the player character where the trappers are coming from, and suggests to aid the player character in retaliating against the attack. If accepted, the settler turns into a temporary companion, and leads the way towards the marked place.

Regardless of whether the player character accepts the settler's help or not, the next objective is to travel to the marked location and eliminate all trappers. Once the marked enemies are dead, return to the settler and tell them the attackers are gone to complete the quest.

Bugs
The quest might activate after an attack is triggered, without any interaction with settlers, which also causes its steps to activate in an incorrect order. The player character is first prompted to kill trappers at a certain location, then report back to a settler. At this point, the quest will become stuck, unless a settler is spoken to about an incoming attack, which will activate the "Help defend" step. Attackers won't spawn since the quest has already advanced to the retaliation step. This issue can be fixed with the console command, completing and removing the quest from the quest log.