Dexter fans were surprised to learn that John Lithgow would be returning as the Trinity Killer, Arthur Mitchell, in the upcoming New Blood revival, but a new fan theory could explain just how he comes back. It was previously confirmed that Lithgow would reprise the award-winning role, but details about his character's return have been kept very secretive. [Please Note: Dexter spoilers below.]
Over on Reddit, a user going by IReadTheReddits shared their idea on how Lithgow might show back up, explaining, "It's confirmed that Jennifer Carpenter will be returning and so will John Lithgow. The theory is that Deb will be Dexter's new dark passenger and John Lithgow will be Harrison's as we know he was also born in blood and who would be better to see [than] the one who killed his mom." In the fourth season of Dexter, viewers were introduced to Miller, a seemingly normal husband and father of two who held a dark secret inside. Miller was the serial killer dubbed The Trinity Killer, due to his ritual consisting of three very specific deaths each time he killed."
Dexter (played by Michael C. Hall) grew close to Miller as he saw the aging family man's life as a possible future for himself. However, things took a brutal turn, ending in Dexter killing Miller but later discovering that Miller had already killed his wife, Rita. At the same time, baby Harrison sat on the bathroom floor covered in her blood. Now that we have some backstory clarity, let's consider the new fan theory: could Trinity be Harrison's dark passenger? The first important thing to remember is that Dexter's dark passenger is not a person and doesn't have personification.
Some have interpreted Harry (James Remar) — Dexter's deceased father, who he would regularly communicate with — as his dark passenger, but really Harry was serving as more of a guide or conscience. The dark passenger is more of a general term used to describe Dexter's need to kill. It was also defined as an addiction on more than one occasion.
With that understanding laid out, we know that Harrison will be part of the Dexter revival, as portrayed by actor Jack Alcott (The Blacklist), but we don't know that Harrison is burdened by the exact murderous needs of his father. Additionally, Harrison never interacted with Trinity, so even though he's often seen as "born in blood" — like Dexter was when his mother was slaughtered in a shipping container — it seems highly implausible that even if Harrison has a dark passenger, it would be Trinity. It is known that Trinity was the one who murdered Rita, so it's possible that Harrison has seen photos of the serial killer and built a version of Trinity to serve as his dark passenger, but it honestly feels improbable.
The prospect of Miller returning is still deeply fascinating, and if we have to speculate on how the most logical option seems to be through flashbacks. What fans might get is a visual memory of Dexter's, recalling his days in the presence of the sinister Trinity Killer. Another possible scenario would probably be a hallucination. Maybe Dexter will picture Arthur in a fever dream or something to that effect. However it happens, seeing Dexter and Arthur in a brooding showdown would be a very compelling moment for the revival. Dexter: New Blood debuts Nov. 7 on Showtime.