The first individual of distinction in this family was Captain Obed Marsh of Innsmouth. Like most of the residents of Innsmouth he was forced to interbred with the Deep Ones, Pth’thya-l’yi. This Deep One ancestry means his descendents will gradually mutate into potentially immortal amphibious humanoids. They had three children, two of whom disappeared under mysterious circumstances. The third, an unnamed daughter, was from an early age educated in France and never knew the dark truth of her conception. The only hint to her ancestry was that her eyes greatly resembled those of the infamous Obed Marsh. Just after the Civil War she married Benjimin Orne and they lived together in Arkham. The two had only one child, a daughter. She at some point moved to Toledo, married and had three children. Two of her children appeared normal, as their father. Douglas however shared his mother's unsettling appearance. He would eventually shoot himself in the head after learning the history of his family. Shortly afterword the mother disappeared, presumably returning to the undersea city of Y'ha-nthlei.
The last of the line appears to be Robert Olmstead and his cousin Lawrence. Lawrence is the son of Walter and was committed to a sanitarium in Canton.
Other relatives are James Williamson, Eliza Orne, Enoch Marsh, Lydia Menize, Barnabras Marsh, Old Olmstead, Mr. Cleveland, Alice Marsh.
- Captain Obed Marsh: The founder of the Esoteric Order of Dagon, and the great-great- grandfather of Robert Olmstead. He died in prison in 1878.
- Pth'thya-l'yi: Obed's second wife and the Deep One great-great grandmother of Robert Olmstead. She currently lives in Y'ha-nthlei, and favors Robert.
- Mrs. Marsh: Obed's first wife and the step-great-great grandmother of Robert Olmstead. Died of illness.
- Alice Marsh: Obed's daughter from his 2nd wife, and the great grandmother of Robert. Died of childbirth. She did not know of her mother's lineage.
- Benjamin Orne: Alice's husband, and the great-grandfather of Robert. Lived in Arkham.
- Marsh Boys: Obed's two sons, and Alice's brothers who disappeared without a trace.
- Onesiphorous Marsh: Obed's eldest son by his 1st wife, half-brother of Alice, and the great-granduncle of Robert Olmstead.
- Eliza Orne: Alice's daugther and Robert's grandmother. She disapperead to Y'ha-nthlei. She favors Robert.
- James Williamson: Robert's grandfather; born in Ohio.
- Barnabras Marsh: Onesiphorous' son, Eliza's cousin and Robert's great-uncle. Nicknamed as Old Man Marsh. Owned the gold refinery. Died during the Innsmouth raid.
- Mary Williamson: Robert's mother and the daughter of Eliza.
- Henry Olmstead: Robert's father.
- Walter Williamson: Mary's brother, father of Lawrence, and uncle of Robert.
- Douglas Williamson: Mary's brother who committed suicide upon learning about Obed.
- Robert Olmstead: The main protagonist of The Shadow Over Innsmouth and great-great grandson of Obed Marsh. Accepsts his fate as a Human/ Deep One hybrid.
- Lawrence Williamson: Robert's cousin who has already transformed into a Deep One.