In case anyone reading this is currently on Kerkvaya SI, DO NOT READ

I am currently running a campaign where the PCs are tracking a famous missing scion of their dynasty and the search led them to Kerkvaya Si, a world colonized a long time ago by an ecclesiarchal colony ship that was blown off course by a warp storm. The world regressed to barbarism after a huge war nearly wiped out the human population a few millenia ago, so now the only tech-savvy group is a pseudo-religious order residing near the site of the colony ship crash. The players are currently there and snooping about. From what they gathered via local legends and the archived astropathic communications of their House, the scion led an expedition to this world and went to a sacred mountain - tribal legends say he then left the world his blessings and disappeared back to whence he came from.

Now, their illustrious predecessor and an expedition he led did indeed come here a few hundred years ago and went to the temples (built around the wreck of the main colony ship), but he did not leave - worried about his expedition undermining their position as prophets and the main holders of technology, the order murdered him and the expedition. They plan to do the same this time too, but the party may have let it slip they control powerful weapons and allies (as in, their ship is in orbit). The PCs also hinted and will no doubt elaborate on the feast that they have plans for developing this world. How do you think the cult would react? I was thinking serving poison, possibly just drugging the RT so he can be captured and made to cooperate. The party will naturally have their chance to smell something is up.

Will this cult act in a unified way? Don't they have factions of their own? Will some plot against the PCs while others will cozy up to them?

The first thing that comes to mind is that the cult can't just murder them straight away. Last time the scion blessed the world before leaving (and to strengthen the cult I would suggest them keeping legends of some miracles at that) so certainly the new "guests" can't do any less. So you'd have the players getting some time to find out something is fishy as the cult tries to get them to bless the world and the cult. Having some off-worlders visit and leave without caring much for the cult undermines their authority so some display of blessed wisdom or might is needed before they can "help the visitors leave".

For the assassination poison is a very effective method (one of the historically best ones at that) but not the most exciting in a game most of the time, since you haven't mentioned it I suspect the players take no actions against this (most do not but one I had in my group ,a deathworlder, was used to everything being potentially lethal and only ate and drank tested food/drink). If you go with poison think about how you make it a fun challenge instead of just a "Toughness test or die". One good way to do this is to make it a series of toughness tests that each impose penalties and are taken in fairly short order after each other, after X failed the character dies, this means its a race against time to get him to the medbay or someone else who can help. Letting them be drugged to faint allows all the usual "prison break" stuff for a campaign so that's allready fun enough. They can also be drugged to weaken them (impose a hefty penalty to characteristics) and then have them ambushed to be killed the old-fashioned way.

But murder can be a far more creative process than just slipping some poison into a glass. Exotic beasts from this world can be good assassin tools, maybe the hyper-aggressive jump scorpions of Kerkvaya SI tend to be used since they often hide in clothes. Maybe archaic defense systems of the colony ship now serve as a death-labyrinth where those that disagree can be tricked to go and perish, all under the watchful monitors of the cult. Perhaps they have a cadre of elite assassins that ensure their rule, and these assassins just got their greatest challenge yet. Maybe the original ship had an archeotech weapon they used to slay the first one and which the cult once again plans on using. There are countless possibilities for murder here.

Will this cult act in a unified way? Don't they have factions of their own? Will some plot against the PCs while others will cozy up to them?

Overall, the head of the cult will act against them, but as I see it, very good rolls on social tests can win them some allies who either know what is being planned or at least can look out for their interest and tip them off in time that something fishy is going on. The PCs were already able to do some good diplomancing among the nomads of this world and come accompanied by a few champions of local tribes and one chieftain-aspirant. They may make the difference in case the cult needs to resort to violence.

For the assassination poison is a very effective method (one of the historically best ones at that) but not the most exciting in a game most of the time, since you haven't mentioned it I suspect the players take no actions against this (most do not but one I had in my group ,a deathworlder, was used to everything being potentially lethal and only ate and drank tested food/drink). If you go with poison think about how you make it a fun challenge instead of just a "Toughness test or die". One good way to do this is to make it a series of toughness tests that each impose penalties and are taken in fairly short order after each other, after X failed the character dies, this means its a race against time to get him to the medbay or someone else who can help. Letting them be drugged to faint allows all the usual "prison break" stuff for a campaign so that's allready fun enough. They can also be drugged to weaken them (impose a hefty penalty to characteristics) and then have them ambushed to be killed the old-fashioned way.

My idea was to use drugs who demage characteristics or cause fatigue levels if the cult has a good reason to want to take them alive. Even if not, it would make sense to drug them a bit before bringing in the really nasty stuff. Alternatively, if the PCs start getting smart to this, a more direct option can be arranged, using the local guards and some special champions the cult's medic have been augmenting.

If it makes any difference, the PCs are only level 2, it is their first expedition in the Expanse.

I would suspect the players' troops onboard the ship, led by an ambitious officer, will load themselves onto landers and take a town or 12 hostage in retaliation. Then they'll waste a few towns with orbital bombardment to show the natives the firepower they command. I don't know if my current players would do that, but I've certainly had that experience in the past.

I was thinking how the cult could best think to remove the RT and his minions or at least place them under control with what they do know, to be honest. How the ship's officers will react if their captain is captured is another story .

Then it's up to the drugs and poisons you make available on that planet and to those people. A voodoo zombie drug would seem to fit the need.