What kind of elements do you find most useful in running or writing a Call of Cthulhu adventure?
For me, it typically comes down to four elements:
The Hook: in Call of Cthulhu, the adventure hook doesn't merely need to get the PCs to the adventure location. They need to be curious and asking questions. My favorite style of hook is to show the players something impossible, which will be explained in due course. I love the three identical dead men mystery from Horror on the Orient Express.
The Hook can be mundane, so long as the players are motivated to start their investigation. Both Agatha Christie and Sir Arthur Conan Doyle had an early story where the inciting incident is a low paid employee quitting their job suddenly and not being seen again.
The Ticking Clock: Any TTRPG mystery is constrained by the amount of time in the session. Keeping clues and plots across multiple weeks is very challenging, though it's the fun of a large campaign like Masks of Nyarlathotep. The narrative should keep moving, so the mystery can be solved before the end of the session.
I personally like time limits to be transparent. "The police will arrive and arrest a suspect in an hour" "the bus comes back tomorrow" or "the cult will perform their ritual in the blasted heath on all hallows eve" If the PCs know how much time they have, they can use time as a resource in their investigation. Time limits can include a random element, like "each hour I will roll a d6, on a 6, the monster strikes. After sunset, additional dice will be rolled" By turning time into a resource, you can add clues which are extremely valuable but time consuming to your mystery. Often forensic clues, or insane scrawlings in a dead language.
The Monster: the monster drives the horror, and can sometimes be obvious from the start. A horror campaign that relies heavily on whodoneit style mysteries often misses out on the value of making a plan to defeat a known adversary.
When I design a monster for a scenario, I like the monster to have clearly defined limits and capabilities. That way, the investigators can learn the loopholes in a being's powers. An example might be a vampire who can alter the memories of his victims, yet leaves them with vivid nightmares of what really happened. This gives the PCs opportunities to investigate with unconventional methods like hypnosis, or to find a witnesses' wife to learn about their night terrors.
The climactic scare: Investigations should come to a climax. In a whodoneit story, this will often come as a result of correctly accusing the monster with evidence. In a howcatchem story, it often comes before that, as the monster lashes out and the investigators get closer. Not every session will have a climax, especially in a more purist game with little combat.
I typically want the climactic scare to be a situation where the investigators might go temporarily insane. The awfulness of the situation should provoke a sanity roll of at least 0/1d6, and going insane should carry a risk of consequences. Often the coherent investigators will be carrying their companions who've fainted dead away or gone catatonic. I often like a horror climax in Call of Cthulhu to resemble the climax of The Silver Chair or Something Wicked This Way Comes, where the PCs get weaker and weaker with each round, and survive only by running for their lives or taking advantage of prior investigation.
What are some of the most important elements for you, and how do you like to run them?
