The Fens Phantom was a prolific serial killer who lived in Boston before the Great War. He was presumed to have claimed dozens of Bostonian lives, displaying them in the Fens street sewer, where he claimed as a 'base of operations' of sorts.
Very little is known about the Fens Phantom's physical features, making it likely he was never caught. What is known is he is male with a deep voice (provided he did not alter his voice in his recordings)
He was a sadistic/psychopathic individual, evidenced by how he displays his victims for the detective working on his case to see. He also leaves holotapes (see: Dear detective tapes) for the detective to listen to as they walk through the sewer, insinuating a love of playing with people's heads.
The Phantom shows heavy frustration with society and societal norms, and how he views people as fake and transparent. He voices this in his second detective holotape:
Fens Phantom: I want you to appreciate your surroundings, detective. I can only be myself in a place like this. Life up there is exhausting. Everyday you smile at people who don't care about you one way or another because that is what you do. Down here I really get to live. I am happy to share it with you today, detective.
Each of the Phantom's displays is accompanied by one or more chalk X's on a nearby wall, apparently to indicate how many victims are in the associated display.
The Fens Phantom is only mentioned in Fallout 4.