I really like the Secrets of New York book. I especially like the "Half-Moon Cult" adventure although it's lack of a box out timeline did make it rather painful to piece together when everything happened or was about to happen. It was also problematic having Mogens and Eugene Vanderklei's stats printed in the Yorkville section rather than in the adventure section. Still it was pretty solid for a one-shot scenario. I've used it to draw James Paterson to New York so I can later start my Masks campaign.
But if you have a player like mine who pokes into everything, you will need to have a bit more information on this cult. Firstly, witch-cults aren't known to include several dozen cultists which indicates that a few things. Firstly there's eight apartments in the facility itself. They're quite small apartments, little more than a cramped room or large-ish office, which indicates that there are eight cultists though they might be hot bedding (sleeping when the other's on duty). I determined that there were six cultist security guards in the facility who were probably younger sons of the cultists and who alternated twelve-hour shifts five days a week with some overlap. These were also the individuals used for retrieval. There's also Albert Day, the chief of security.
In truth anyone in the facility needs to be either a cultist or controlled in such a way that they might as well be. I assumed that the cook was a cultist, placed the nurse as Eugene's wife, and then included the two assistant scientists on the payroll. It explains that while the facility is pristine there just isn't much experimentation going on because there's not enough people to really get it cranking. Besides which, in occult experimentation scenarios there's very rarely a fleet of scientists helping each other. It's normally just a few mad geniuses having flashes of insight.
Mogens is a cultist. I put in the claim that he had a wife just as all of his ancestors have had a wife. In truth it's a lie and a cover for when he recreates himself as his own son. I see Mogens as an aromantic asexual. His interests lie elsewhere. This is mostly because giving him a family or anything similar just adds complications. His home security guards are regular people who know nothing of the occult. They do know that their employer is scary, however, and will kill them or their family members if given reason to do so. This means that they don't need to be slain to end the cult.
So we have Mogens, Albert Day, Eugene Vanderklei and his nursely wife, Vanderklei's two assistants, six security guards. I decided to limit the coven at twenty four + Mogens. That's a nice manageable number who could attend ceremonies in Mogens' home (as described in the book). The others are all dilletantes, media moguls and key businessmen. A husband and wife team of dilletantes are Mogens' heavy hitters and know a few spells to compel others to do their bidding and then forget the memory afterwards.
The next main question is: What occurs at the ritual? The adventure assumes that the only natural choice is to head to the laboratory but there is an equal chance the players will want to straight to the source and attack Mogens. Is this a good idea? Hell no! But we need to have some idea of a ritual to make it work as well as some objects and religious paraphernalia that can be found locked away in Mogens' bedroom. If you look at Mogens' spell list, you'll see that he can create a number of items, so those should certainly be present. What else?
So we have a New Moon Cult. Areas of light and darkness will matter. Black and white ritual table cloths. While the other cultists wear full black, I like the idea of Mogens wearing a half black / half white suit, like two vertical halves of a suit sewn together, with a tie that shows rather disturbing imagery of Nyarlathotep, a coven and some sacrifices. A tie that causes 1 sanity loss to any who pays too much attention to it. A glance at it while talking to Mogens is fine but a closer examination - enough to give a full description - is a 0/1 sanity hit.
So that covers some of the decorations. But what could they spend their time doing during their rituals? A eugenics cult of business people doesn't really work for ecstatics and, besides, I have the Bloody Tongue cult for that. I wanted something sedate. So I see them interlinking hands in a circle around the ritual chamber, intoning strange syllables that feel like the rising and crashing of waves across alien shores, and whose sounds create flickers of imagery in one's minds - imagery of alien worlds and bizarre cityscapes and possible futures of humanity. Just brief flashes, nothing more.
Mogens leads them in this prayer and then once per month they take a sedated homeless victim of another race and lay him upon a thick round table made entirely of folded bandage-like material and wrapped in a white sheet, perfect for soaking blood. The victim is both sedated and given a paralysing draught but the sedative is a weak dose designed to keep them in a half state for the start of the ritual when each participant strides forward to carve a moon-shaped line in the victim's flesh while the other's intone. By the time Mogens must take his place to scoop out each of those twenty four circles with a melon baller, the victim is fully aware but unable to scream.
Out of the shadows behind Mogens coalesces Nyarlathotep in his avatar of the Black Man and the shadow seems to thrum and vibrate, not fully drawn into our reality (because that should not be such a regular occurence) but vaguely there, and it reaches out and pulls the essence of the victim into itself and leaves the victim's corpse in a state of terrified rictus with a multitude of round cuts from its body. Then, and only then, do the intonations begin to drop away, the shadow dissipates, and they all retire to one of the other rooms for a brandy.
The servants are worshipful and devout to Nyarlathotep but aren't allowed to be true cultists. There's a housekeeper, maidservant, cook and general dogsbody.