In the Victorian Era, there have been strange bouts of ritualistic happening, both in and near Great Britain. Of this, you know nothing... or do you? What we know: Strange murders with strange crime scenes. "They appear to be bizarre sacrificial homicides," one newspaper will say. "And seem to be the work of possibly some type of cult activity throughout northern Europe, primarily Britain, Germany, France, and Ireland. There also appears to be an aquatic theme revolving around the strange group, primarily associating with oceanic mollusks, primarily octopi and squids. Eye witness accounts report the sightings of aristocratically dressed men, with what seemed to be octopi-like masks." Spoiler: What the secret is. (For people who want to be cultists ONLY.) You are the servants of our Dark Lord, Great Dread Cthulhu. That's pretty much it for that, so, enjoy being a cultist. Mystery, action, adventure. Puzzles? That's for you to find out.
Can I be a cultist of a different deity? They would have basic knowledge of the Cthulhu cult (aware of their existence, but very little knowledge of their secrets and rituals), but would be working against the cultists.