So recently while running a game of Dark Heresy one of my players concluded based on the available evidence that the individual they've been tracking could be a 4th generation genestealer. They explained to the other players that even though they are Ordo Hereticus it's important that they make sure to confirm this detail. Listening to this deduction I immediately conclude that their theory is really cool and that it would make for a crazy twist.

However…

This is not case in my notes. The clues in the investigation just so happened to cause a player to come to this conclusion. I'm tempted to change things on the fly but a part of me recognizes that solving the case can be very rewarding. If I change the imporant evidence of the case on a whim it's kind of a breach of faith that it "could be solved".

What do you think? Cool to switch things up so long as they're none wiser? Or would you call it railroad GM fiat?

(Also bonus question)

I know a bit about genestealers but I don't know all the details.

4th generation genestealer (so nearly completely human)

1. Do they have to be bald?

2. Genestealers are psychic and can be directed and controlled by the patriarch. Does this mean they don't have free will?

3. Could a 4th generation genestealer be unaware that they are a genestealer? (Say the patriarch is thinking deep cover)

Avoid railroading if you can, as it breaks immersion.

You probably need to study up on genestealer cults before you can make a decision, I cant remember it in great detail, but its one of those things that has been around since the beginning of warhammer 40k. So you should be able to find resources on the net, like

http://wh40k.lexicanum.com/wiki/Genestealer_Cult

I also think there is an article on it in creatures anathema or was it the rogue trader xeno book? I have to look it up.

Rereading my original post I felt I could be more clear.

For the investigation the suspect being tracked is not a genestealer. One of the acolytes thinks it could be. I think it might be cool if that was true but unsure about ethical implications of changing the case on the fly.

Personally I would consider throwing in some red herrings to make it appear as a genestealer hybrid/cult, but personally I wouldn't change whatever your current plot is. If there are similarities between genestealer cults and whatever it is you actually have in mind, I might play it up.

Consider what methods of investigation the party might use while determining hybrid status. I think in other books, its been considered to be quite hard to medically determine the hybrid status of someone without fairly invasive surgery. Try to come up with interesting, but suitably vague and paranoia inducing results.

I would weight the "difficulty" of genestealers vs. whatever plot you have in mind, and go with the worst. If the players think its genestealers, but then find out its a "harmless"cold trade smuggling ring, then I would either up the smuggling rings status to "freakin terrifying" (e.g. Amaranthine Syndicate, give em some Slaugth buddies), or switch it over to the genestealer cult.

Hopefully whatever you have in mind is suitably terrifying, to make the big reveal even better ("Oh, you thought I was a genestealer? How cute -insert evil grin- I gotta remember to tell that one to my buddy Khorne when we go out for golf next time").

This doesn't sound like railroading to me - in fact it sounds the opposite, changing your plans to suit the players rather than forcing them to follow your plans. I wouldn't worry about anything 'unethical' about switching the hidden details mid campaign if it makes it cooler. If it's easy enough to do the switch, and you think it would make the campaign more enjoyable, then go for it.

As far as I am aware, a genestealer hybrid does have free will to a certain extent, although the magus or patriarch could control them with telepathy, most of the time they will do their own thing. However their overriding priority is to protect their own kind and to aid the cult's expansion. It would be difficult for a genestealer hybrid to not know they are 'different' from humans, but it's possible they don't know they are genestealers. The majority of genestealer cultists are completely unaware that they are the vanguard of a tyranid invasion, and that once the 'sky mother' they worship comes they will be absorbed into the hive fleet.

Genestealer hybrids are universally bald, but I could imagine one having hair implants to better integrate with the society they are infiltrating.

Quidam,

Definitely appreciate your help with the genestealers. The hair transplants idea sounds is clever.

KommissarK,

I think for my dilemma you nailed this on the head. Which ever provides more shock value is the better choice! While I plan on working in some genestealer stuff I think it might ultimately distract from the tyrant star (the big bad in my game). The acolytes don't know that the person they're tracking is leading them to the source of global insanity.

Depending on how you direct your game, I would say that you shouldn't change what you're doing; there is nothing cooler for players (depending on their mentality) to think they've understood what's going on and then discover that this is even wronger thant they thought at first.

In a game, I used methaphorical descriptions speaking about bones and chitins to describe their adversairies; which just appeared and disappeared in ambushes; the players thought these were tyranids and that was why the ship they were investigating was destroyed, but it was not; these were warp spider (armours in wraithbone, elongated limbs, etc.) and when they understood that they were facing supreme intelligent, fast and highly technological adversaries, they became very affraid. You can turn anything into horror if you use them well and you create a sense of ignorance in your players by your descriptions.

But there are many factors; the personality of your players, the real potential of the horror you want them to fall in; if your big bad evil is less horrifying than a genestealer cult, maybe you shouldn't let them fall on something that isn't as bad as they though.

There's nothing wrong with changing details to suit the flow of the story. In this case I would stick to what was planned already. Let the players learn the lesson that just because they think one thing it doesn't mean that this is actually so. This can also easily come back to bite them, hard, in the ass in a later adventure.