First off: if you're playing in a campaign in Darmstadt, stop reading.
My players have behaved very well in the last adventure, satisfying the wishes of their inquisitor to the fullest, so they got handed a new mission with a LOT of power and responsibility: take control of a light cruiser and determine if the population of a planetary system that has recently resurfaced after a millennia-long warp storm is fit to be incorporated into the empire or if it has to be quarantined. The Tech-Level is comparable to the middle ages. This are the facts the players know.
My plan is to have them encounter a society that is spread over a single, big continent, with about a hundred different power-groups, some heretical(but not chaos-following, just fallen from the imperial creed, following strange gods or totems) but mostly fit for integration. The sole exception is a city-state that is described by the others as run by an evil council of oracles that control mind and soul of the population.
The plan is to have the city-state being under the influence of enslavers. When the threat first became apparent millenia ago, an inquisitor sealed the whole system with a warp storm, so the enslavers never got reinforcements. Afterwards the inquisitor waged a war against the enslavers, largely successful, only the one city-state escaped his wrath, mostly due to the fact that the industry of the planet was shattered by this war and most of the population killed. The planet slowly rebuilt to a level of blackpowder weapons. The other states fear the city-state and appeases it with human sacrifices, sometimes this city-state wages wars without provocation, generally the other nations just try to stay out of it's way because apparently everyone who enters the city-states borders falls to their evil ways. But since the warp storm ceased, the enslavers pull in more and more enslavers, making them always stronger. Unbeknownst to the population the enslavers have used this reinforcements to established warp-gates in the other states.
The players have to figure out what the enslavers are and how to deal with them.
If they use the light cruiser to wipe out the city-state, the enslavers will re-emerge in other states, carefully obscuring their presence until it is too late. If they try a ground assault, their forces will succumb to the masses the enslavers control. Infiltration will be almost impossible since the enslavers know everything that happens on their turf, taking over those that dare to cross their border.
The inquisitor that last sealed the system in the warp storm was buried in a crypt, this crypt has been lost but the players can learn clues to it's location from the other states. In the crypt they will find detailed informations about the enslavers and arcane devices to help combat them(maybe even the inquisitor in a stasis field?).
That's my idea so far, haven't done much more. Any comments?