So, I am wondering if something would be possible, stupid, or what have you, and thought I'd see what other people think. The Tyranids are often very paradoxical, in my mind. At the same time, they are mindless, slavering killing machines, but there's a genius-level intellect involved. They are both direct and sneaky, they are both immediate gratification and long-term planning. How feasible might it be for a Genestealer Cult to form on a ship? Many vessels stop and pick up new crew at various planets, far more than can be checked over. There are many people aboard a ship who few people ever see, many clannish folks who keep to themselves, and live and die their whole existence within its confines. There are also many nice, dark places for them to hide. If the Cult could get a nice foothold, and then enslave the top crew, they could have a mobile nursery ship, filled with food and bodies able to support implantation. At some points, many of the infected could still look Human, and so stopping at worlds wouldn't be too difficult, and a nice opportunity to seed other locales with branch cults.

So, could this happen? I know that the Nid presence in the Expanse is supposed to be negligible, and the Hive Fleet is already in the Reach, so maybe Genestealer Cults aren't necessary there, but under the right circumstances, when the Imperium's systems are lax in checking, or not strong in enforcement, could it happen?

It definitely could. Genestealer cultists and hybrids could easily pose as crew, buy their passage or simply sneak onto a ship. By almost all the fluff i've ever read, 40k ships are full of places to hide and could easily support a small, hidden population of genestealer hybrids. It would be relatively easy to take over a ship as well, when compared with an entire planet. The only problem (from the hybrid/cultist point of view) is that if they are detected early, there is nowhere to run.

Personally, I'd say go for it! It's an interesting idea that could also make for a brilliant Dark Heresy or Deathwatch plot.

The main problem that I see would be that travel time in the Warp is a bit unpredictable, so you may have to handwave travel times a bit in order to ensure that the "infection" can actually spread within those generations that actually appear human - but with a fast ship it should be possible to visit a dozen worlds within only a few months, provided you don't run into any phenomena.

Another issue is that there's only one Patriarch, but here you could just say that he "knows" (thanks to inherited racial knowledge) that another Patriarch will be born in a cult that does not already have one, kind of like the Queens of the Alien franchise, and so he has no problem with this idea, and with sending the best-looking carriers off to some world in order to intermingle with the local populace and spread their "gift".

Also, there is this bit about the Genestealer Cult being genetically programmed to emit a psychic signal to call in the nearest Tyranid Hive Fleet. However, as it's been said that this only happens once the infection has reached a "critical mass", you could simply assume that just a single ship (as opposed to an entire world under 'stealer control) is not nearly enough to trigger this beacon.

To make it appear more likely, the initial infection of the ship should perhaps not have been a targeted effort but simple coincidence - a press gang clubbing an infected civilian (who was just minding his business, impregnating others) unconscious and dragging him or her off to the ship, where they awake and find themselves confronted with a new environment ... and over the next one or two decades, the infection spreads throughout the ship, with the Purestrain Genestealers hiding in the lower decks, until finally a rebellion overwhelms the remaining humans and the cult takes control of the ship.

I've played around with this idea myself a bit.

Both with the idea of simply having a genestealer cult hidden away on the ship, but also with the idea of having the entire dynasty be infected (and probably have most of the PCs related).

The second idea I have yet to impliment. The first... now that would be telling,

Yeah, my players have found, and "rescued" a pilgrim vessel, the passengers of which (and key crew members) were members of a 'stealer cult. They weren't paying attention and sold everyone into slavery without noticing the hybrids. It wasn't until they found some stasis caskets (not all occupied) that they realised they'd been dealing with genestealers. At that point, the human-looking and mostly-human-looking ones had already been shipped off to slavers on Footfall (yay for knockout gas). They tried to sweep the ship to get rid of the patriarch and any purestrains, but kind of got loads of their mercenaries killed in doing so. So they left it out in a parking orbit, shoved on the plague lights and had the ship's atmosphere dumped. All of it.

Obviously, that is not guaranteed to kill the purestrains, but it will force them either back into their stasis caskets, or into hibernation until some prey gets close/the atmosphere is restored. But, y'know, it's not like it's waiting in a heavily trafficked system with a strong scavenger culture, right? ...Oh. Yeah.

And as for the enslaved cultists, well, I'd ruled they were just below the critical threshold to start calling. They were without a purestrain/patriarch to act as a focus, but there was also a high proportion of 4th gen hybrids, so the next children they had would be purestrains. So, it's only a matter of time...

It took the characters quite a while to figure things out (and the players almost as long). The initial response when they did was "We can't go back to Footfall" followed by "This is why we can't have nice things"

There are actually a couple of good threads on Genestealers on this forum already that I'd recommend reading but I don't see any reason why the Death Cult on a ship couldn't actually be a secret genestealer cult instead.

Here are some threads worth looking at.

http://community.fantasyflightgames.com/index.php?/topic/81097-genestealer-cult-game-idea/

This one is a must as it has great Genestealer rules and the idea of a rogue trader using the genestealers as a way of strengthening the bloodline: http://community.fantasyflightgames.com/index.php?/topic/93575-rules-for-genestealer-hybrids/

a) Genestealers aren't mindless killers. Genestealers are being guided by the Hive mind and are specifically bred to be inflitrators as well as close combat troops. Their intellegence is the bit that really makes them dangerous and not representing them like that is a mistake. (Though in most stories on the subject majority of people seem to be contractually obliged to THINK that the genestealers are stupid animals. Much like in every Alien franchise movie, comic and novel.)

b) I think some novels comment that killing the Patriarch genestealer is a temporary solution and another one will grow to become the patriarch unless the brood is destroyed so I believe that when the brood sends out members to other ships/planets the new brood will have another Patriarch grow as soon as it is big enough.

c) Genestealers don't just look for new targets in far away places, they also weaken the defenses in the immediate targets of the Hive fleet. So having some arrive few years or generations before the main fleet (even if it already was targeting that planet) is a good way for the Hive fleet to scout their target and weaken it a bit to ensure a quick victory.

d) I am a GM in a RT game and having a genestealer cult attempt to infiltrate the RT's ship is one option I have thought about. It might also be a Chaos cult of course, but depending on what my players do I have a few plans and use what seems to be the most appropriate.

Something one should also keep in mind is that genestealer broods tend to subvert and use the Imperial Creed for their uses.

I think it was in the 4th edition codex, where they described how people actually lined up to be taken in by the tyranid "savior" bioships.

Imagine a family of genestealers acting like missionaires only to plant their seed and then a hivefleet shows up and gets the full buffet without having to lift a limb.

Genestealers are not under the direct control of the Hive Mind until a Hive Fleet arrives in system, possibly even orbit. They are engineered to function without its guidance to an even greater extent than other Tyranid vanguard organisms, like Lictors.

A Genestealer Cult/Brood are bonded through some sort of group Telepathy instead, this makes them extremely coordinated and able to execute complex plans perfectly. Although this telepathy is family based, meaning other 'stealers and hybrids from another family does not share in it, at least not at first.

And yes, Genestealers tend to be very intelligent, often functioning above average human levels. They are also patient. A cult with a Magus or Patriarch in the lead is a very dangerous thing - something you do not want on your ship

Genestealers are not under the direct control of the Hive Mind until a Hive Fleet arrives in system, possibly even orbit. They are engineered to function without its guidance to an even greater extent than other Tyranid vanguard organisms, like Lictors.

And yes, Genestealers tend to be very intelligent, often functioning above average human levels. They are also patient. A cult with a Magus or Patriarch in the lead is a very dangerous thing - something you do not want on your ship

Genestealers are not under the direct control of the Hive Mind until a Hive Fleet arrives in system, possibly even orbit. They are engineered to function without its guidance to an even greater extent than other Tyranid vanguard organisms, like Lictors.

Even more so than lictors, as I understand.

Thats what I said

It has even been mentioned that Genestealers tend to flee when the Hive Mind approaches so as not to be subsumed by it and eventually shoved into the Digesting Pools.

Wasn't a hulked ship, located outside Footfall in the orbiting asteroid field, taken down by a Genestealer cult on the ship? And if it was that close, whose knows where they came from/went.