Robert Montauk was a serial killer associated with The People's Church of the Divine Host.
Robert killed more than 40 people over a 5-year period (1990-1995) for Maxwell Rayner. He killed or at least dismembered them in his backyard shed, leaving only the hearts in jars of formaldehyde. In MAG 143, Manuela reveals that Robert killed at first to "feed the beast" in the hopes that his wife, Linette, could still be saved. However, he later started sacrificing cultists to the beast to try to banish it.
He was finally caught when he murdered a member of The People's Church of the Divine Host and a creature affiliated with The Dark (presumably "the beast") attacked Julia, prompting her to dial 911 and then flee to the murder shed. Robert apologized to Julia and then ran as the police sirens approached, leading to a manhunt. The bodies were never recovered.
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This article contains information from later episodes of The Magnus Archives, and may contain major spoilers for the setting and plot.
Imprisonment and Death
Robert was caught in early spring 1995, after Julia called the police while being attacked by a Dark-affiliated monster. After Robert's arrest and trial for the forty murders he committed, Montauk was imprisoned at HMP Wakefield. He was kept in the most secure and unpleasant wing of the prison, but he generally kept out of trouble. Julia began visiting him in prison soon after she turned 18, and Robert lied about his conditions and told her it wasn't too bad in prison.
In fall 2002, a man matching Maxwell Rayner's description visited Robert. The two men appeared to despise each other, and when the lights went out, Rayner said, "You didn't think you could kill it for long, did you?" As Rayner was being led out, Robert shed a tear. He became lethargic and demotivated after that visit.
Following a blackout at the prison on November 1st, Robert's body was found in evident dismemberment in his cell, though no one else had been in the cell.
Although Robert killed for The People's Church of the Divine Host, he does not appear to be an avatar of The Dark. Instead, he may be affiliated with The Hunt, which seems to ally (often unwillingly) with other entities, as in the case of fellow police officer Alice "Daisy" Tonner.