For starters i need to share the party rolled death cult for complications(again) and i thought that due to thier recent "luck" fighting of chaos fleets 3-4 times it would be kinda cool if they started dealing with the taint sreading throughout their ship and eventually their fleet.

For setting it up they will eventually be able to track the spread of the taint back to a lower deck officer who also happens to be a leader of whats now a splinter faction of the deathcult. He decided to keep a sword trophy from one of the boarding actions and its corrupted him. His eventual goal is to shut the gellar field off mid warp to sacrifice the whole ship "In Nominae Imperator"-really a chaos deamon whispering in his sleep

My question for you good folks is this "Any suggestions on how i can hint and slowly build this heresy up to something they must deal with?"

Ive already been taking extra crew pop off and random drops in population and ive been reporting lower officers in charge of random stuff missing for the day. But how can i turn up the heat without bringing in the inquisition just yet? I will bring the inquistorial hammer down if they ignore the signs long enough but i cant figure a step between random missing reports and being under investigation because they keep leaving little heretic cells all over the void.

P.S. This cell has been subtly working for 1-2years in game.

If your party has any astropaths they might start to sense something/ go crazy/ eyeballs explode- granted that last one is not that abnormal, but still.

Have one of the more high ranking cult members transfer to an npc ship that then goes missing in the warp, 2-3 small ships would be ideal if your fleet can spare it

start rolling higher on the warp encounter chart/ making test to avoid things harder/ roll far more often then they should normally have too with no apparent reasons

start having members of the clergy ( if your ship has any) go missing

Have the tech-priest notice / catch someone messing with a gellar field and avoided disaster barely and the culprit killed themselves rather than being caught while spouting some gibberish Nominae Imperator.

​You could also have the sword physically start to taint the ship, while the cultist see it as shining gold or something, it could in fact be rusting away or any of the other chaos tropes.

Have a chaos ship(s) meet them in the void, but instead of attacking right away have them call the ship and start talking like they were supposed to meet up.

​somehow your latest round of orbital "negotiations" keeps taking on the shape of a chaos rune/ always leaves craters in one of the sacred numbers etc.

​Have some rumors reach them in port about a minor warlord who has the same name as one of the more major NPCs that have gone missing.

or don't let the crew on until it is too late to turn back, then introduce them to a little book called Black Crusade.

Does it really has to be the gellar field of the ship? Anybody who has actually been LIVING on a voidship must (in my opinion) be totally nuts/ravening mad to come up with this as being "a góod idea". Regarding the Inquisition.... you are on an RT void ship. The Inquistion will only have third Hand accounts of what your ship is REPORTING to be doing, unless somebody is taking testimoney of your Crew/Ratings at port and reports it further...which will turn it to hear-say.

That being said, back to your request.
How about a heigtend amount of "hulghast hunts" in the lower decks? The characters will see it as a growing hulghast Problem (hulghast = void-hardened mutants living in the black holds/sealed decks of ships). Fact is... the hunts are a lie. The lower deck officer in reports hulghasts to be able to order hunts to get Hands on weapons and ammonition. At each hunt, he loses one or two Hands (cult members, then hiding away themselves) together with their weaponry. His plan is to have a dozhen dedicated members for an assault. Other steal Food stuff and of course the ammo "used" in the hunts is given to hidden cell members. If someobdy gets assigned to the hunt that is not part of it...and cannot be fooled..well....People die on hulghast hunts, if things go bad.

That so many hunts happen, with los of men but without being able to produce even ONE head of a hulghast (they say they have killed or deadly wounded something, but lack proof..actually, they killed nothing) could lead to the command Crew taking note...and taking Charge. After all , the low deck officer seems incompetent, his "Claims" of successful hunts lack proof..obviously, he tries to cover up how inempt he truly is.

...well, what happens if the characters start to score the nearby black holds themselves? It is not like they will find a Thing...the deck Crew is up and alert...there ARE hulghast...but when question, everybody just recalls somebody else having said they have seen it. You catch the drift

Hmmm, This is all really good stuff. they have no psyckers among the party but there is an Explorator. The Hulghast thing is really nice i hadnt even thought of that. most of the time they just leave internal problems to the provists and to reasonably well to let the provists handle problems without trouble. Currently they keep returning to imperial spaces via a trade route they hope to open between the Lathes and a run down mechanicum installation(lots of Archeotech and lost knowledge stuff) they found but the various factions are making it hard to nail down a contract. so the Inquisition has had plenty of chances to sneak aboard and or grab a rating or two for first hand accounts. I dont know that i want them to go that far as to become Chaos renegades, and unfortunately they can spare the ship getting lost to the void. but that isnt necessarily a bad thing that makes it more likely they will notice being one ship short. I think i will start turning up the void encounters for no reason they can discern. Thanks for the advice. Next session isnt for a week or two ill report back with what i do and how it works. cheers guys.