I was just playing Fallout 4, got a request from a settlement to clear out Dunwich Borers, at first I thought 'Okay, cool I'm clearing out raiders' then 'oh, a horde of feral ghouls and visions, likely due to some kind of gas in the mine' only to get to the bottom and discover a ritual and ancient Eldritch entity trapped under the mine/quarry... best settlement quest I've gotten so far.