Call of Cthulhu is based on the writings of H. P. Lovecraft and others, which began as several friends who were playing a behind the scenes game of referencing each other and their works, and their own other works. Simplest example, not sure which order you read them in, but when you were reading the second of the two stories, At the Mountains of Madness, and the Shadow Over Innsmouth, when you ran into the word ‘shoggoth’ and you remembered it from the first one you read, that sweet chill was hard to ignore, whether or not you’d call it fear is irrelevant. The Cthulhu Mythos is not necessarily the first ‘shared world’ but among the first in modern fiction, all the more remarkable because it was done without any formal thought behind it. Even the writers themselves didn’t necessarily think it through in advance, they just connected their stories and stories of others when and how it suited the story they were writing and their mood at the time of the writing. This is one of the reasons there are inconsistencies, but they are the kind of inconsistencies that make it better. Later developments in the post-Lovecraft years created some disagreements, which are still discussed at times, in various forums and media. When the game was created it set aside some of the more ‘controversial’ of these elements, but some Keepers bring them back in. Fair enough, every campaign is the collaborative effort of the Keeper and the players, and as long as everyone enjoys it, all the better.
But the bulk of the Mythos is the result of a shared world concept, and it has its canon, and the game was built around it. We all know Arkham Massachusetts with its witch filled history, and good old Miskatonic University, the Orne Library, Professor Armitage…the region of Dunwich, the town of Innsmouth, the stretch of road known as the Aylesbury Pike. These are parts of the background of the game, and even if you use a different setting, it is in the background, it is something you can run into, in some sense or another. Even if you make your own version of Arkham for your west Texas setting, or a town to fill Arkham’s place that has nothing to do with Arkham per se. This is part of the background, and part of the world you create.
And as Chaosium began to create campaigns and scenarios, things started again. The shared world concept began to creep in. Carl Stanford, one of the ‘men behind the curtain’ in Shadows of Yog Sothoth has a guest appearance in Masks of Nyarlathotep, and even if the characters are not the same ones, if the players are, there is a moment of ‘oh hell I know that guy’. When reading At Your Door, you run into the NWI logo, players who were run through Day of the Beast/Curse of Cthulhu/Fungi From Yuggoth will pause and feel a chill. Players who dig deep in their research and find out who Thalassa Chandler, the current CEO of NWI, in the scenario Coming of Age from the Unseen Masters collection, will feel a chill. Finding that information may be beyond the scope of the scenario itself, but if they do learn it, there is that extra gem. So, a separate canon has grown, one based on the scenarios and stories that followed. We have Mister Shiny from Michael Shea’s amazing story Fat Face, who guest stars in At Your Door, as well as lurking in the background elsewhere (and in the 6th edition, I was never able to relax and decide if the Shiny we met in one example was supposed to be the same…which made his actions all the more chilling.).
That last parenthesis is part of the point. When you have players who you play with regularly, keep in mind the Chaosium canon in addition to the Lovecraft canon, even in the modified form of your campaign.
To paraphrase infomercials….But Wait, There’s More.
Your own campaign will have events and characters that have impact. So when the next campaign starts, the next set of characters, whatever, remember those moments. That policeman who was never quite helpful enough in the first scenario can return to remain a foil…or a foe….or an ally. That villain they never quite beat the first time returns. Or the company he worked for proves to return (I am real sneaky on that one, I will borrow company names from movies and stories and watch the players struggle to remember where they’ve run into it before- examples include the Watershed Corporation, Banodyne Industries, and the housing development Vista De Nada).
Another thing, and this was one I picked up from an interview with Sandy Petersen, the man who made the game. He mentioned that the successes and failures can have impacts on the game world, impacts that may not be something that ‘John Q. Public’ can accept or understand, but may not be able to ignore. He cited an example off the top of his head of a Great Old One manifesting on the world leaving (if memory serves) Chicago never having sunlight fall in it again, regardless of time of the day. This is not something that people can ignore, but no one can explain.
So let your second campaign have something like this if you have room for it (in other words, the players managed to ‘hold off the catastrophe’ but it was close enough you can put something like this in the game world. A river that always has the color and smell of blood, a city that cannot be entered during daylight, or where time runs backwards, or citing the Chicago reference above, where during the day you can’t find Chicago, and if you enter it, the moment before dawn immediately is followed by the moment after dusk, the players jump from one moment to the next as an example. Some place in the game world where the previous game leaves its mark.
So you start with the two canons, you add your own…and the game grows, and your players get those extra little jolts when something familiar, but disturbing, creeps into the picture.