Not because it’s Halloween since I’m not American, just because Caladan being mostly a water world just screams out for a mysterious cult who sacrifice to their underwater god by drowning the unbelievers.
Thinking about running it as a bit like a CoC scenario, mostly investigative once bodies start floating up in greater numbers, all marked with an odd brand or sigil. Rumours of strangers abducting people from the streets - those people later found washed ashore.
String it out over a number of sessions, perhaps cultists have infiltrated the player’s House or operations…
Eventual showdown with cult leader once their temple is found. Leader obviously mad but is there some interference/guidance from another outside House?
Obviously there’s no squamous-or-otherwise-tentacley god in the deeps but a lack of an actual god has never stopped any religion…
Still mulling over motivations etc but it could work.
Maybe there is no god below the surface, but indeed something, more mundane, that needs some cover-up? Read: Someone does something underwater that needs to stay unnoticed. To ward off fishermen etc. they start this cult that initially just killed people in the way of the operation. Due to some internal schism (or something along those lines) things get out of hand.
Could be the influence from another outside house you were looking for. Maybe it’s a listening post, maybe a sabotage operation (covered up by another sabotage operation), maybe a scientific or mining operation, discovering and/or exploiting some scarce resource under the sea. Or maybe troops landed on distributed locations (to cover up movements) are brought to the location via submarine and prepare for an invasion of some kind.
External manipulation of the locals is always fun and a classic Dune plot.
Though turning it internal can be used to throw the players off balance. Caladan strikes me as the sort of place small, insular villages are fairly common.
It doesn’t take much to twist the plot of The Wicker Man to your service here.
I can also imagine the House investigators turning up and being treated the local delicacy of lobsters.
They head out the following day to do some investigating and find a dead body crammed into a lobster pot… Might make them queasy about their last meal.
Oh, actually I think it’s both! You see, the cult leader is aware that the cult is fake and only a cover-story for something else. Yet, obviously, outside forces still manipulate them to obfuscate the nature of said something which is really something else.
Maybe, the cult leader even thinks he is lead by (some of) the characters, speaking, if interrogated, in detail about instructions he says he received over the last weeks or months on conspirative meetings. He knows some actual quirks of the characters by heart and is generally familiar with them, as if they were allies. Maybe he even knows about hidden scars or past adventures, something, only close friends would know.
Of course, the characters only come to interrogate the cult leader as some other members of the cult also know this, saying the information was revealed to their leader by their vile god…
And also: Are we the persons behind the curtain and we just don’t know it? Or did someone make us forget? Maybe even said god? Did this god trick us to believe they do not exist and the cult is real?
Maybe the victims of the cult are drowned (“sunk” in the cult’s own vocabulary) because they did not let the vile god of deep insight sink into them. The teachings might say that deep insight and great wisdom flows through the waters, personified by the cult’s god. But not all let the god fully into their selfs. So, according to the teachings, they lose memories and start to act out of their usual patterns, do all weird things as their minds try to suppress the evil inside and battle for control… Obviously, these heretic brains must be punished!
The players, trying to uncover everything, need the cult leader for their information. It comes handy that the cult leader thinks the players are all behind it. But by beginning to investigate, the cult leader grows a spark of suspicion. Afterall, the players now act exactly like the cult leader teaches their followers to uncover heretics that must be offered to their vile god. Power, they say, is like sea water. The cult leader might question whether the characters are tricking him to believe the cult isn’t real – while it maybe is! In parallel, the characters will obviously distrust the cult leader and their own memories.
Maybe, I should write a Cthulhu-story out of this, could be fun.
I’d add you might like to try adding a layer of folk horror (Appalachia or spooky English village type of thing). The nobles are quite removed from the ordinary folk who could have all manner of strange customs and rituals to bless the harvest or bring in the new season.
Actually, I recall a Fading Suns campaign from years back where I had a recurring villain in the form of a mad Doctor who was a Clone Arranger - he’d illegally cloned the local evil Baron. While he was driven off the planet by the PCs (and the deformed clone torched) he’d keep popping up again and again with a new (and improved!) version of the Baron to hound the players.
Not sure I’d get away with this in Dune - could be a crazed Tleilaxu Master and his legion of gholas - it was more of a comedic shtick which would elicit eyerolls from the players every time the Baron would reappear…
In Fading Suns at least there were some body horror elements I could utlise for the ‘changed’ - not really so applicable here in Dune. Still, never say never…
To tie things back into more mainstream Dune ideas, the cult could be the result of a Missionaria Protectiva operation gone feral. It might have been slowly mutating from its original purpose to the bizarre murderous cult it is now over decades or centuries.
And obviously the Tleilax could easily be the creators of Deep One-ish monstrosities, for any number of unpleasant reasons.