WARNING: The following contains spoilers for Dexter: New Blood Episode 6, "Skin of Her Teeth," which aired Sunday, Dec. 19, on Showtime.
Of all the mass murderers and serial killers Dexter Morgan has squared off over the years, none have been as thoroughly terrifying as the Trinity Killer. Serving as the main antagonist for the original Dexter series' fourth season, Arthur Mitchell was a seemingly unassuming family man who had secretly been killing victims in twisted sets of three that led him to earn the nickname the Trinity Killer. And while Dexter himself had killed the Trinity Killer by the end of Season 4, the notorious murderer's legacy continues to haunt Dexter and his now-teenage son Harrison as the revival series Dexter: New Blood nears its finale.
Prior to being killed by Dexter, Arthur had discovered his pursuer's true identity and murdered Dexter's wife Rita as his final victim. This was done before Harrison when Dexter and Rita's son was still only an infant, with Harrison discovered on the scene by Dexter covered in his mother's blood. The tragedy mirrored Dexter's own origin story, with a young Dexter similarly witnessing the murder of his mother. And while Harrison initially appeared relatively well-adjusted when reuniting with Dexter at the start of New Blood, in the small upstate New York town of Iron Lake, the Trinity Killer's influence is felt years after Arthur's death.
Harrison was steadily showing signs that his upbringing was mirroring Dexter's, from boasting a genius-level intellect to having a secret dark side that leaned towards violent outbursts fueled by a twisted sense of justice. Harrison was visibly haunted by what the Trinity Killer had done to him and his family, listening to a true-crime podcast exploring Mitchell's bloody history ending with Rita's murder and carrying the straight razor Arthur used for his murders. And as Dexter finally has an overdue heart-to-heart with his son after suspecting he was quietly following in his footsteps all along, Harrison admits the truth: He vividly recalls witnessing his mother's murder at the hands of the Trinity Killer.
Harrison's confession comes as he has grown closer with Iron Lake's local business magnate Kurt Caldwell in clear disobedience to his father's wishes as Dexter senses Kurt's deadly true nature as a secret serial killer himself. With Harrison having nearly killed a classmate and broken a rival wrestler's elbow at a high school competition even after he yielded, the impressionable teenager is visibly struggling with his dark side. And as he tearfully reveals this to Dexter, Harrison notes that his violent impulses are fueled by the rage and helplessness stemming from his traumatic memories of Rita's death, with the incident and Trinity Killer appearing to him in his dreams.
Dexter's most serious attempt at creating a normal domestic life for himself was completely upended by the Trinity Killer and this tragedy continues to create longstanding consequences for him and his son Harrison. Arthur Mitchell may have been personally killed by Dexter to bring his crimes to an end but the family of his final victim has become twisted and haunted. Mitchell personally felt that his and Dexter's fates were intertwined, and, judging by Harrison following in his father's footsteps, this distinction has covered over generationally. Dexter still hasn't confessed his own murderous secret to Harrison which could stand the two men closer together even as the source of that shared pain caused by the Trinity Killer looms overhead.
To see how the Trinity Killer's legacy remains intact, Dexter: New Blood airs Sundays at 9pm ET/PT on Showtime.