I have homebrewed and run a lot of Call of Cthulhu cosmic horror adventures, so I hope my thoughts, experiences, and perspectives can be of some use to you as you examine and refine your own gamemaster strategies for running investigative cosmic horror RPG sessions.
Conventional mystery stories are the most highly structured narratives in all of genre fiction—just ask anyone who has every had to write one, tried to write one, or wanted to write one. The stories are typically structured into four acts rather than three with upwards of a dozen identifiable stylized plot beats (sometimes as many as 28(!), depending upon what standard compositional template you follow). The archetypal structured mystery story is the ubiquitous murder mystery, which when viewed most simply can be organized around the timed drop of “the three corpses”—the inciting incident for the investigation, the redirecting setback, and the revelation that locks the protagonist and antagonist in a climactic battle—and the events circumstances surrounding their demise.
Unless you’re running a Cthulhu funhouse dungeon full of overwhelming Mythos monsters popping out of the shadows to eviscerate your underpowered player characters one by one without warning, your cosmic horror RPG adventure will be primarily investigative in nature, and the story narrative you present to your players to explore will have much in common with traditional mystery investigation fiction in terms of its organization and structure. The bad news is that there usually needs to be many more moving parts that the GM needs to track and advance than in the typical ICRPG heroic fantasy quest, requiring greater planning, precision, and attention to detail for the adventure’s narrative to hold together. The good news is that even though the GM has a lot of prescribed responsibilities behind the scenes in an evolving mystery story, to the players your RPG mystery investigation can still feel like a complete sandbox if you do your part. The other good news is that you can still have Mythos monsters pop out to overwhelm and eviscerate your PCs.
I realize that the ICRPG community favors strongly a “less is more” bullet-points-only approach to preparing very straightforward “timer-threat-treat-GO!” encounters and adventures, but for the reasons mentioned previously, putting a little more thought, effort, and planning into your cosmic horror adventure will pay big dividends at the table. With the addition of some simple genre-specific mechanics and tropes, ICRPG is an excellent engine that supports cosmic horror roleplay quite well, and its streamlined mechanics and general ease of use for GMs designing playable content will help you create space within your allotted prep time to pay attention to the needs of your investigative narrative. If you lean into some of the the similarities your cosmic horror adventure shares with mystery fiction, the story you develop and lay out before your players can be uniquely entertaining.
Shifting to bullet points, here are three helpful things to prepare for this sort of genre adventure:
Clear stakes—Define in your own mind not only the identity and motives of the major NPC actors in your cosmic horror adventure narrative but also the nature of “the next big thing” that they are trying to accomplish; this is usually what the player characters will try to discover or foil.
A detailed timetable—Mysteries have an intrinsic timed aspect to them; they are a race against the clock before some undesirable consequence occurs. Have a well-developed temporal framework that describes in sequence the steps your antagonists are taking to advance their goal. A simple calendar or timetable of things that will happen in-game independent of your player characters’ actions unless they provide specific opposition can really help you define and present an evolving game narrative that feels real and engages your players.
Key locations—Have a short list of significant venues for the players to visit or interact with the NPCs in your game. Depending upon the structure of your adventure, you may organize this list into a point crawl matrix with nodes and connectors, or you may simply enumerate all of your “prepared spaces” with descriptors, key features, and clues listed in association with them. This is where you install your “timer-threat-treat” architecture. Note that not all of your significant venues are likely to be known to your player characters at the start of their adventure, but through investigation and discovery, new portions of your sandbox’s “matrix” will be unlocked, or they can be presented to the players at key points in the narrative as defined by the adventure timetable.
People who write mysteries generally start the construction of their plot in one of two places: the endgame or the inciting incident. Authors who start with the endgame envision the antagonists ultimate goal and the circumstances surrounding it, and then they work backwards to define all the prior steps that were required to achieve that goal, identifying the points where the sleuth could have obtained a clue or intervened. Authors working from the mystery’s inciting incident follow the same process of defining necessary steps toward an ultimate goal, noting branch points for contingencies where the sleuth and the antagonist cross paths (directly or indirectly) as they “discover” the eventual path of their story narratives. Either method can serve you well as a GM prepping for a mystery adventure.
The last point I will offer for your consideration involves the tone and scope of your investigative cosmic horror adventure. Experienced mystery novel fans develop considerable meta-knowledge with regard to the structure, tone, and tropes of the genre fiction they enjoy. They know to glean subtle insights from the scene of the crime of the first corpse, they anticipate the sleuth’s call to action, and they expect a midpoint setback. As the GM running a cosmic horror investigation for your players, get them prepared for the atypical set of experiences this RPG genre offers. This is not a dungeon crawl—and even when it is, it does not go down the way it does in classic fantasy settings. Make sure they are apprised of and willingly invested in the nature and tone of the kind of game you are running, so that they can play along in informed fashion with expectations well met; this can be telegraphed early in-game or stated explicitly. Most of the time, when players stumble into overt combat in a cosmic horror mystery investigation, mistakes have been made and things have gone horribly wrong. Demonstrate to them early on that noticing, gathering information, and acting with forethought and purpose are their most powerful weapons to combat the dangers ahead. An NPC’s lie during a social interaction can be as deadly to the party as any pit trap. Cosmic horror investigation is a different RPG landscape full of its own perils, and most of the time the odds are stacked against the PCs. Have fun inventing and setting up these new kinds of obstacles for your players.
Hope that helps… Viel Glück!