Right now I'm designing an adventure for a solo campaign that's been on hiatus for a long while - mostly because I painted myself into a corner. I've since convinced the player to let me retro a chunk of it so that I can make it far more manageable. It's a relatively high consequences game with a fair amount of realism for a Fantasy game and therefore it's not impossible for me to make it too difficult for his teenaged protagonist to get involved due to me making the issue TOO big and TOO epic for one person to logically deal with or at least affect in a meaningful manner.
So we retro'd a chunk of it and the huge city-wide party has become a much smaller party in a large house that allows me to introduce some gothic horror in the style of Alone In The Dark: The New Nightmare mixed in with alchemy and other sorts of ceremonial magic. I'm about to show him why living in a magocracy based on sorcerers can be a bit of a problem and why these blooded mages are so against ritual magic that can theoretically be used by anyone.
It helps that his character belongs to a cursed bloodline of nobles who pre-date the magocratic revolution that occurred 90 years ago. Each member has a curse that is connected to their own personal traumas and vices that erupt over adolescence so we'll get to explore that aspect of his character as well. It also adds some extra Gothic zest to the household where this party is at as they are also Rosentias and also a part of this.
Anywho, so right now I am building up the mystery from the bottom. During this I did a bit more world building to figure out how many nobles would be in the city so I could get an idea of demographics at the party. While doing this I realised that as these mages and their blood relatives (which make up the nobility here) are big on universities there'd likely be a higher percentage of nobles in this urban area due to the universities. So I researched and generated three universities and a Medical Institute which has given me an edge on the Gothic mysticism meets science elements I need.
I've also been researching for the Mysticism on the Home Front and the core supernatural elements of Horrors on the Home Front and this has given me a much better idea about alchemy and folklore, albeit from a primarily Western perspective. I'll doubtless end up reading more as the adventure continues and that'll give me a better idea about other elements I can include.
So the next thing I've started to do is populate the party and generating key individuals. Most of the Happening will occur once most of the guests have gone home as otherwise there's too many NPCs to handle but during the party there'll be important clues dropped during conversation. Most of these clues will be in gossip and rumors but some of it can also be found in the entertainment, food, and decor of the main ballroom.
I look at some interesting characters of his age (children between their 13th and 14th Midsummers as one's 14th Midsummers is one's coming of age day where one rates as a youth rather than a child) and consider which of the five Houses they hail from and how they interact with each other. Each House is more of a mega-corporation of similar personas who were elevated into the nobility during the revolution. For example, House Ansalon is the house of marine families (pirates, explorers, naval captains, etc.) and scientists (secret alchemists, dyers, and known scientists) who assisted the revolution. Each House has two allied houses and a rival house. Each House is nominally allied to the ruling House Audor who were the original sorcerers and such-like who led the revolution. These are the basic political framework that I need to bear in mind.
Then, of course, there's the tangle of where individual families' allegiances lie. This province is administered by House Rosentia and the Lady and Lord (basically appointed mayoral couple from the noble families, though they use hereditary titles because it makes it easier for the common masses to understand and respect the hierarchy) are the ones who have set up this party. So that means that even some members from Rosentia's rival house (the PC's rival house) may be friendly with the PC. After all, they wouldn't be invited to a Rosentia party if those individual families weren't on good terms with the Rosentias.
We add to this the closed world of teenaged politics that has been developed by, well, children still and things become all the more complicated. There could well be overly polite, covertly vicious falling outs that turn best friends into arch nemesis or vice versa. The shifting sands of pre-adolescent to burgeoning adolescent (as these kids generally are) psychologies and politics are bound to bewilder the rural perspective of an outsider like the PC who was raised a manor servant in his island of origin.
So I start writing down names. And allegiances and enemies. I brainstorm a list of possible explosive moments and other awkward moments. I put in encounters about well-meaning kids who get shunned and arrogant kids who get torn down and popular kids that dance the line between nice and mean and the strange kids who no one wants to upset nor befriend because they have poor social skills but are quite clever and therefore might become important people in this academically oriented political world where inventions, study, ambition, charisma and magic interweave to create your political position.
He's been told this party doesn't matter by his father to help him relax into it. His father was lying.
I also start writing down rumors that will matter later on. Things the PC might learn about the key figures who will begin the party encircling the ballroom with the other adults to ensure that the children can't cower by the walls like most wallflower parties of strangers at that age (remember any mixed gender school-based party you went to at 13? Everyone clumped together against the walls). I'll throw in a few red herrings about unimportant NPCs who will be gone by the time it really heats up.
Then I start jotting down the entertainments. There will be clues there. A sign of a love of exoticism. Plus it gives me a chance to show off what a sorcerer's gathering might well look like. Two musicians from the Ihlander Salt Plains which plays havoc with unborn children so that their various pigmentations might vary considerably from the parents (alongside other, somewhat benevolent mutations) where one is a dark-skinned woman and the other a white-skinned man (this country is olive skinned and dark haired). The children must dance. The PC has had intense training over the past couple months but is still a rural servant boy at heart so there's risks here. The dances themselves are different in style to ours although they do bear similarities to certain types.
Finger food will be laid out. Mostly exotic dishes from a number of different countries to show off the Lord's frequent expeditions overseas and to faraway places. Perhaps a few alchemical spices to reconnect it with magic. A good talking point for some of the clues and a good way to help the player understand that this is a fantasy world he's dealing with.
I'll need to think up a few more entertainments to summarise though I think eating, dancing and gossiping should about cover it. Then most of them will go home and the rest will go of a tour of this mansion of exotic curiosities where they'll hear some folklore and start getting the sense that something is amiss. I'll brainstorm some signs and then collate the best of them for use.
Finally, we'll come to the main event! A corpse is found during the tour. Is it a murder or something else? The mansion's defences come into action to fortify the location but its off-kilter and no one can get out. It'll be up to the PC to piece things together while avoiding or attacking the dark things that wander this strange mansion.
Long story short, the various clues will have been laid by people. I was entirely confused by how to go about this until I remembered that what matters is what the people are doing and who they are and what they'd leave behind rather than focusing on the 'event'. The event is comparatively boring. It's dangerous but it has no history. The people involved in it have a history and their history has left a lot of marks on the mansion and those marks are the clues that the PC will follow.
So remember the people involved when you're generating the mystery. Remember them and follow their steps and let it all slide together.