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On the Threshold contains examples of:
- Agent Scully: Phil seems to be one of these, often dismissing evidence of the supernatural in favor of more mundane explanations
- Alien Sky: Either this or Space Clouds might describe what Dr. Applegate calls "the cloudscape" in her dreams after they go wrong complete with a massive unseen predator stalking her from below
- Body Horror: One hopes the thing Matthew encounters on his second delivery wasn't once human
- Brown Note: Creating these seems to be Zoey Evans's goal
- Butt-Monkey: Constable Wiggs is repeatedly confounded and publicly embarrassed over the course of less than a week. Adaman evades his arrest in front of a crowd, and later wrestles him into unconsciousness in front of the town's children. He's later tricked by Alexandra into letting the Traveler family go with legal nonsense, has his prisoner jail broken and money from his personal vault stolen, and finally is brutally burned to death after he attempts to arrest Adaman.
- Cosmic Horror Story: "A Descent into Cosmic Horror" is in the subtitle after all, and so far we've seen glimpses of incomprehensible horrible worlds and abominable occult rituals
- The Coroner Doth Protest Too Much: Dr. Powell thinks professor Albertson's heart attack is the result of poison, not only because Dr. Powell suddenly experienced a near heart attack the same night, but because the next day the results of their research together was ransacked
- Courier: Matthew in "Cost of Delivery" is a driver for DoorDash, and two of his midnight deliveries are the basis of the episode
- Creepy Cathedral: A VR example in The Cathedral of Bar Shachath
- Deal with the Devil: Implied to be what Tiago Cabral was trying to achieve. Whether he succeeded is unclear.
- Eldritch Abomination: Possibly whatever "the Maimed Angel" is, but we know little about it at this point, and it might just have been an insane delusion by Tiago Cabral. Also, probably whatever dream-borne predator is stalking Dr. Applegate.
- Eldritch Ocean Abyss: Invoked with the imagery of Erasmus's visions
- Escaped from the Lab: Kickstarts the plot of "Civilized", even if we don't know it until halfway through
- Even Evil Has Standards: Allegedly, even his neighboring slaveholding Brazilian plantation owners wouldn't help Tiago Cabral put down his slaves' revolution, out of disgust for his horrid rituals and experiments.
- Facial Horror: The thing in "Cost of Delivery"
- Fallen-on-Hard-Times Job: Dr. Powell considers Alexandra's legal consulting to be this, as actually taking a service job, let alone a woman doing so in Victorian society seems demeaning for an aristocrat.
- Festering Fungus: Dr. Powell suspects that the release of a long-dormant fungus in the caverns is the cause several character's insane actions and Adaman's almost supernatural ability to influence people
- Laser-Guided Amnesia: Seems to be a likely explanation for much of the missing time experiences by Matthew, Dr. Applegate, and at least some of the subjects of her study. Phil is skeptical of most of the claims and rejects out of hand any connection to UFO's
- Healthy Country Air: Dr. Powell prescribes this to Erasmus and at least one other patient, taking advantage of the former to come along and give himself a vacation. Considering that the alternative is the smog of Victorian London, this probably isn't a bad treatment.
- Horror Hunger: Implied in "Cost of Delivery"
- Humanoid Abomination: Whatever it was that Matthew encountered on his second delivery in "Cost of Delivery
- Human Sacrifice:
- Implied to have been part of Tiago Cabral's horrific rituals
- Erasmus has a vision of human sacrifices, which Dr. Powell compares to the Real Life human sacrifices by the Carthaginians
- Frequently referenced throughout the "Civilized" episodes, but whether it happens during the story is unclear
- Mad Artist: Combined with Mad Scientist Zoey Evans regards says that "the human mind is the ultimate canvas on which I paint my art", which means she's intent on creating novel mental experiences through her VR artwork, one of which includes an altar with the corpse of Jesus Christ as a preamble and is intended to cause the "inversion of religious ecstasy", which she says leaves those who experience broken and shunned if they ever express it. The fact that many who have tried the VR environment have seizures doesn't concern her in the slightest, and indeed just seems like proof that she is close to her goal.
- Medical Horror: The experiments in "Civilized", complete with drill sounds
- Missing Time: A common element in many episodes
- Impoverished Patrician: The Brownes, though it doesn't stop Erasmus from being an Upper-Class Twit and pushing every responsibility onto his sister
- Never Sleep Again: Dr. Applegate is implied to try this, unsuccessfully
- Or Was It a Dream?: It's unclear whether the entity stalking Dr. Applegate's dream is real in some sense or just a persistent part of her dreams. Either way, it seems to be affecting her health.
- Ominous Knocking: An element in Dr. Applegate's recurring dream.
- Killed Mid-Sentence: Played with. Dr. Powell's narration suddenly cuts off mid-sentence, but he resumes it to say that he's just nearly had a heart attack which he only noticed and prevented due to his medical training.
- Playing with Syringes: The experimental neurosurgeries carried out in "Civilized" on Disposable Vagrants and Erasmus, the exact purpose of which is still unclear, but Erasmus considers himself one of the lucky subjects to "only" occasionally slip into fugue states in which he experiences hallucinations (maybe) of alien beings and places
- Phony Psychic: Dr. Powell assumes that "the Traveler woman" is one, but it's not actually proven.
- Psychological Horror: Much of the horror revolves around the questionable reliability of characters' minds, like Dr. Applegate's suspiciously-specific amnesia and sense of alienation from pictures of her from the time she no longer remembers.
- Recurring Dreams: Taken to the Logical Extreme and Played for Horror in that after Dr. Applegate discovers her own work, she literally can't seem to dream about anything other than getting into a looping conversation with herself and wandering empty hallways. Breaking free of the cycle temporarily only makes it far worse.
- Sealed Evil in a Can: An unusual and ambiguous example. Dr. Powell theorizes that a fungus was long dormant in the caves under Castle Bolsover, and that their sudden release was the cause for many characters' rather unhinged actions
- Secret Underground Passage: Accidentally discovered in episode 5 underneath Castle Bolsover, leading both to the base of the hill on which the castle sits and deeper into the strange underground lake
- Slave Liberation: The successful revolt of Tiago Cabral's slaves
- Spooky Séance: Happens in episode 5. Dr. Powell assumes it's a hoax, but this isn't actually confirmed.
- Stable Time Loop: Part of Dr. Applegate's recurring nightmare, in that her conversation with herself through a locked door leads her to leave her office and get lost in the hallways until she finds her office and tries to get in, until she hears herself in her office...
- The Shrink: Dr. Applegate
- The Strongman: Milon Adaman seems to be a 19th century version, as the discipline was just starting
- Tome of Eldritch Lore: Probably A Revelacao Preta and its translation The Black Revelation, considering that it's derived from the notes of an insane and sadistic plantation owner who tortured his slaves to try to appease and contact demons. Unusually, it seems to have a practical use aside from the occult, as Dr. Castle considers it to be a useful comparative anthropology between the practices of shamans in the Amazon and West Africa. She suffered feelings of being watched, missing time, and not long after a death under questionable circumstances, though, so the ominous pattern still holds.
- Upper-Class Twit: Erasmus, though his family falling on hard times (no thanks to him), seems to have complicated this somewhat
- Victorian London: The setting of the first half of "Civilized"
- You Cannot Grasp the True Form: Erasmus struggles to explain what exactly he experiences in his altered states. When Dr. Powell suggests that he explain it in poetry he seems better at explaining it metaphorically, but even still Dr. Powell is confused by the descriptions.
