I don’t know why, but this whole mini-adventure was way harder than I thought it would be to write, and I’m not sure it’s up to my usual standards of quality. I can’t point to any specific flaw, but it all feels very off to me.
Summary: Shadow people have possessed the entire town. Several shadow people without hosts still remain, and will attempt to possess anyone who enters.
Discovery: Any character can make a Very Easy(+30) Psyniscience check when in town to notice that the place is going absolutely bonkers with Warp activity. Any Inquiry checks will turn up that there is nothing at all suspicious going about around town, however a Challenging(+0) Insight check made when asking any townsperson about suspicious activity for any reason will reveal that they are hiding something. Whenever characters are in the dark (including at night), they automatically make a Challenging(+0) Awareness test, and if they succeed, then they can see faces staring at them from the corner of their eye. Any effort to turn and confront the face causes them to fade into the darkness. They can only be seen out the corner of your eye.
Exploration: Any psyker who establishes a telepathic link to any of the townspeople will immediately discover that there are two minds to read: One a half-lunatic townsperson with no control over their own body, either raving or utterly resigned to their fate, and the other likely thinking “I sure hope they don’t notice that I’m not a regular human.”
Any townsperson interrogated will insist that they’re just an ordinary person unless a character confronts them with the specific knowledge that they are clearly some kind of possessing spirit or if they are subject to some kind of torture. In either case, the shadow person will exit their corporeal form when this happens. If the interrogation is happening in a room that contains at least one human-size patch of pure darkness (including things like shadowy corners), all characters present immediately make a Challenging(+0) Awareness test, and if successful, they see the shadow person out the corner of their eye in the darkness. The freed townsperson will babble incoherently about a miracle of the gods or not believing that it can really be true for a few minutes or until threatened or harmed, at which point they’ll cooperate entirely with anything the characters ask of them, including information. The shadow people arrived several years ago, and systematically possessed the entire town.
The shadow people themselves don’t generally know anything about the greater plan. They’re just weak Warp entities who want a free ride through the material world and are happy to enjoy the food, wine, and physical intimacy of a corporeal body. The town’s mayor and Red Guard sergeant are the ringleaders, and are scheming to find some bodies to unload their remaining shadow person buddies into. They can’t leave the cursed perimeter of the town, but their physical bodies can, which means any shadow person in a possessed body who tries to exit town will only free their host from the possession, and must instead wait for others to come to them. Being something of a backwards town, this opportunity comes rarely. However, there are only a few shadow people left in town, specifically, one less than the number of players.
All of these details are unlikely to be revealed by a psyker scan, because shadow people have no reason to reiterate what they already know (like the reason they can’t leave the town in search of a few more hosts) and will instead be dwelling on the problem they’re trying to solve: How to possess the PCs and what to do with the spare.
The remaining shadow people will follow the PCs everywhere they go and make periodic attempts to possess them. The PC must make a Routine(+20) Willpower test to resist possession. If they succeed, pass them a note telling them that they saw a figure lunging for them out the corner of their eye, felt a brief sense of overwhelming dread, but were able to fend it off. This character will not be targeted for possession again for at least 24 hours, but doesn’t necessarily know that the possession failed. If they fail, pass them a note telling them that they have been possessed, are now aligned with the shadow people, and must avoid leaving the town at all costs as that will end the possession. Different shadow people have different personalities, so it doesn’t really matter exactly how the player pursues their new alignment. They can ignore the alignment switch completely, in which case they’re acting as a deep cover double agent, or they may try to lure the other characters away from actions that might harm the shadow person conspiracy.
Confrontation: Shadow people possessing a corporeal body will die when the body does, so they will abandon any body that appears to be in mortal peril. If the entire town is destroyed and subsequently quarantined, the shadow people will be trapped inside, unable to affect anyone outside of its limits. If the characters have the allegiance of either Grey Harbor or Imberkavitas, or can convince either of them to attack the town based on the results of their investigation, the shadow people can simply be bombarded into oblivion and the town’s former grounds quarantined to prevent any shadow person who thought to depart their current host during the bombardment from acquiring another.
Characters who have learned that the town is cursed can attempt a Challenging(+0) Forbidden Lore (the Warp) check or a Difficult(-10) Forbidden Lore (Heresy) or Scholastic Lore (Occult) check to know the proper ritual to uncurse the town, which will drive all the shadow people from it at once. This requires a dozen people of powerful will to make a Difficult(-10) Willpower check at certain points along the perimeter, while a thirteenth with the Banishment psyker power must succeed on a Hard(-20) Willpower check. Attempting the check takes a full action for each character involved, and attempting it over and over again until everyone comes up successful is fine. It will likely take several hours in-game, but if the characters can gather up enough people who can plausibly make a Difficult(-10) Willpower check (NPC priests and especially sorcerers are likely candidates), they will be able to destroy the town’s curse and immediately fling all the shadow people from their corporeal forms. Of course, the shadow people are unlikely to just sit around and watch while this happens, and while they cannot cross the boundary to directly attack the ritualists, those ritualists are standing just a few centimeters outside the boundary and the shadow possessed Red Guard are perfectly capable of shooting them.
The limits of the shadow people’s influence is city limits, not the entire hex, so characters could also drag any possessed party members out through the perimeter, then just leave the town and give it a wide berth in the future. Any character depossessed in this way (or any other) takes 1d10 Insanity from the possession.
The mayor can also be persuaded to let the players go in exchange for providing him with other hosts to possess. This requires only a Challenging(+0) Charm or Deceive check (depending on whether they’re being honest), as there’s nothing particularly special about the characters’ bodies from the shadow people’s perspective. Indeed, if one of them is Mechanicus or Heretek and has replaced significant chunks of their flesh with bionics, the would-be possessors may well prefer a more pure host. If the characters can serve up a suitable batch of hosts (absolutely any host will do so long as it’s physically healthy and not too old), the mayor will offer hospitality to the characters in exchange. At this point, the town is effectively a vassal to the characters, and it’s treated the same as though they had installed a friend in power over the town.
If all the shadow people end up possessing a character and only one is left, it’s time for the dramatic reveal wherein you announce that it’s time to tie up this loose end and kill the straggler. This isn’t very likely to work, just because that straggler almost certainly has at least one Fate point to burn and even characters so committed to their betrayal as to spend or burn Fate to kill their party member can’t actually do so to override the “get out of death free” use of Fate burning. As such, the one remaining party member is virtually guaranteed to escape the immediate ambush. At that point, they must find a way to unpossess their friends while working alone (or more cynically, escape the town and call in the artillery), while the rest of the party attempts to hunt down their former comrade. After the troublesome character survives the initial ambush, one of the shadow people with a body will be ordered by the mayor to give up their corporeal form, who will be promptly shot, so they can track down and possess the final unpossessed character.
If the worst should happen and the shadow people end up winning, i.e. all characters are possessed or killed, the surviving possessed characters are imbued with excess Warp energy by the mayor, allowing them to exist beyond the cursed limits of the town for several weeks at a time. They must still make regular return trips every 30+1d10 days or else they will be depossessed. The mayor tells the characters to go out and spread their curse across the land. Their very presence outside the bounds of the curse will make the curse hungry to catch up with them, and slowly expand past its original limits. At this point, if the characters would like to continue playing their current party as shadowy possessees, they can do that. Otherwise, they can roll a new party and their possessed former characters can show up periodically as antagonists. In both cases, it is possible that the characters may end up all depossessed over the course of the campaign.
The shadow people can also be fought in the same manner as all incorporeal creatures can, and unlike some of the region’s ghosts, they don’t even have any effective attacks outside their once-a-day possession trick. The problem is that it is impossible to focus on them. They immediately and automatically vanish into the darkness when you turn to face them, and in light they cannot even be seen out of the corner of your eye. This means that characters can catch their approximate location when they succeed on an Awareness check to catch them out the corner of their eye and can make one immediate attack at a -10 penalty. Other than this, they are fighting completely blind.
Rewards: The shadow mayor has very limited occult juju. He can invest power in a half-dozen-ish shadows to go beyond the limits of the town exactly once, and only after every shadow person has already possessed a host. If the characters befriended the village without being possessed, they can ask the mayor to use this trick to have some troublesome bodies possessed by more cooperative shadow people, who can then be empowered to leave the town and resume their former lives elsewhere (though they will have to make periodic trips back to the town to avoid being depossessed).
Once depossessed, the townspeople are going to spend several hours borderline hysterical, but will fairly quickly become a regular town. Befriending the shadow mayor will also cause the town to act mostly like a normal town. It grows crops. It produces products. It puts those products on a barge and ships them out to other towns in exchange for money. All of this can be taxed, despite the fact that it’s a bunch of shadow people. As such, whether depossessed or possessed, ruling the town is worth the same 1d10 Influence for direct rulership or 1d5 Influence for installing a friend (including another party member) as all hex encounter towns, and as usual it can be levied for 2 Influence to automatically succeed on one requisition check on locally available goods or gear. The town can provide anything that’s common or cheaper, any solid projectile weapon or service that’s average or cheaper, and any medical care, low-tech weapon, or basic armour that’s scarce or cheaper.