Speak to Cassie in Far Harbor to start a series of tasks avenging some deaths in her family.
National Park Campground
The first of three steps in this quest is to eliminate the feral Ghouls at the National Park Campground. You can either focus on the marked targets or clear out the entire area of Ghouls. Whatever you decide, return to Cassie when you're done.
Your next stop on your tour as an avenger is to take down a Fog Crawler at Dalton Farm. Defeat the monstrosity and report back to Cassie. She will bequeath the farm to you, and you can now use the workshop at Dalton Farm.
Brooke's Head Lighthouse
The final Dalton death to avenge takes you to Brooke's Head Lighthouse. Head southwest of the town where you can find the lighthouse not far from the quarry. There, fight your way through Mirelurks, Trappers and Fog Crawlers. Make your way up the metal spiral staircase to the top of the lighthouse to find Douglas, who continues the Trapper tradition of great headwear.
Loot Douglas' body to find a Mysterious Holotape. In the same area, find an issue of Islander's Almanac on a bookcase. This particular issue offers you a 5% higher VATS chance against animals you are in combat with.
Optionally, in the same area, look for Douglas's key in the bathtub and for a note by a Brahmin Skull. Walk down to the docks and check the bottom of the right net for a trunk for some ammo. Return the holotape to Cassie, and she will reward you with Bloodletter, a hooked melee weapon where targets bleed for 25 points of additional damage.