Blood Tide

During the quest Living on the Edge, you'll receive an optional task of talking Cassie Dalton. Talking to her will start this quest. She'll ask for your help in "getting back at the island" for taking away her family. For the first chapter of getting revenge, you have to locate and wipe out the feral ghouls that killed her cousin.

Head to the National Park Campground further west of Far Harbor. Eliminate all feral ghouls in the area then report to Cassie. She'll give you 200 caps for the trouble but you can ask for more. Continue talking to her learn the next of her kin that needs avenging.

Next, you'll have to go to the Dalton Farm, further north of the island to hunt the Fog Crawler. You’ll find the Fog Crawler when you get near the farm. It is very tough and dangerous as well so consider engaging it only when your level is high enough and you have ample weapons and protection. When you’re done slaying it, report back to Cassie to update her quest. You’ll receive 200 caps and the Dalton Farm as your very own settlement in the island.

Creepy crawler

Finally, travel to the southern end of the island to reach Brooke’s Head Lighthouse. The trapper leader, Douglas is found at the top of the lighthouse itself. First, clear the enemies around the compound then make your way to the top. Loot Douglas’s body to find a Mysterious Holotape. Play the holotape then scavenge the place for more items. (Refer to the location page for the lighthouse for more information)

Report to Cassie once again to complete the quest. If you played the mysterious holotape, you can ask Cassie about it. You’ll also receive 500 caps and the Bloodletter weapon.