I've been wondering about the effect of racking up Corruption when under the spotlight of an Apothecary. The rules are not specifying any mechanical effect, and in theory Marines are resistant to outright major mutations until they reach the 100 Corruption cap. On the other hand, Apothecaries can specialise in reducing Corruption (the Guardian of Purity Special Ability), and the Red Scorpions Chapter, obsessed with biological and genetic purity, also have some abilities and Squad & Solo Modes working towards the same. Now the Librarian's similar abilities can be easily explained by the extraordinary mental resistance they supposedly possess, but the Apothecaries are focussing on the biology and genetics, so it's harder to handwave it away with Warp- and sorcery-related wibbly wobbly timey wimey mumbo-jumbo.

So I guess my questions are these:

According to fluff, the members of the Apothecarion (both PC and NPC) are routinely testing the Brothers for (genetic) purity. Is there any chance they might catch the scent of Corruption in Marines - perhaps somewhat similarly to the Mind Probe and Reading Psychic Powers? If yes, what might the limit of the probing be? And what for the narrative effects? For example, what happens to a Marine caught with some minor genetic level irregularities/mutations? Please note I'm not necessarily planning on homebrewing a comprehensive crunch guide for Corruption, I just want to a) keep my more corrupt players on their toes and b) allow my rather reserved Apothecary player a chance for greater immersion and understanding of his role.

I always felt that in Deathwatch - perhaps due to a lack of GW background detail - the Apothecaries are demoted from their Codex-approved authoritative and sacral role, on par with the Librarians or Chaplains, to a battlefield medic and part-time science guy. This is somewhat understandable: genetics is hard science after all, and a basic understanding of it is part of educational curricula in most parts of the world, so if GW or BL or anyone delves into too much detail on the topic, they quickly risk breaking the immersion and suspension of disbelief. It's just I find it slightly unfair to take away the majority of combat spotlight from the player and not giving much in other fields in return.

The finer details of the Apothecaries' role are also glossed over in BL publications - I can't recall any (loyalist, non-HH) novel where an Apothecary is the main hero, and the story revolves around his roles and responsibilities, save for one ancient short story, Apothecary's Honour from the Dark Imperium anthology, and that, too, is a relatively straightforward gene-seed extraction story (with a twist visible from miles). I like to compile reading lists for my players to improve immersion, and there are complete cycles about Librarians and Chaplains, but there's virtually no meaningful Apothecary presence. Does anyone here know about any useful GW/BL/other 40k material?

Also on a slightly related note: did you have any Apothecary as a player, who instead of going for the +1d5 in healing, rather picked the toxin-creating or purity-related special ability? How did it pan out?

For the sake of full disclosure, especially if someone from my group wanders here: I'm handing over the GM mantle to a guy in my group to have a couple of months off to rest (and of course plot) a bit, and as a player I'll bring a Red Scorpion Apothecary (Purest purity of the pure is pure! Catholic Space Nazies for the win!) and I plan to play it straight - my group is rather mature, generally willing to compromise, which is generally a good thing (outside 40k I mean), so I wanted to introduce a PC to be a thorn in their side in his unbridled hatred, limitless contempt, hardline stance and unrealistic expectations. That still means I'm looking for answers for my future campaigns - my current PC will be feasible as it is, but getting some info would provide some enhancement there too.

I hope this all was comprehensible. Sorry for the wall of text, but this line of thought bugs me for some time - all feedback would be greatly appreciated.

My players are actively trying to avoid crossing over to Black Crusade territory, so there's no problem with motivation it's just my gut feeling as a GM that collecting Corruption should have consequences , however minor they may be in-game, otherwise PCs become desensitised to its threat, and staying within boundaries could cease to be a choice, turning to be a necessity of game mechanics instead.

The xenos contamination angle is a good idea; I'll definitely start using it in my campaign, thanks for pointing it out - it was definitely something I searched for. It should have been obvious, too - it's even mentioned in the Apothecary Speciality description in Core (p. 68), meshing together nicely with the WD quote above:

"Aside from monitoring the state of the Space Marines’ implants, the Apothecaries must also be ever alert to the risk of alien contamination. [...] There are some xenos that actually reproduce in this vile manner, their seed infecting the victim and slowly mutating him into the original life form * . Such blasphemy must be guarded against, and the Apothecaries administer an entire bank of tests on every member of a Kill-team before and after every mission ."

That still leaves us with the Corruption issue open though, and my problem with Warp-based corruption – or other genetic drift for that matter (see: Blood Angels) – as a narrative element is that it’s hardly a reversible process (stabilizing the course being the only obvious source of temporary relief), and thus it’s difficult to find a middle ground between “60 CP? Test results came back clear, carry on, soldier” and “60 CP? You’re irrevocably tainted, you and your Chapter must bear eternal shame, and after we dispose of the body, we throw your gene-seed into the incinerator with a pair of tongs, which we also burn afterwards”. Mental corruption, on the other hand, could in theory be diminished to some degree by undergoing cleansing rituals and the like (I'm aware that from a mechanical, RAW point of view Corruption cannot be undone in DW, but that's besides the point now).

Semi-important addition: I'm planning to utilise the Removing Corruption section from DH2 (DH2 Core, p. 291): "The insidious touch of the Ruinous Powers represents a permanent taint on the soul, and is normally impossible to remove. It is this permanence that makes Chaos so difficult to combat, for once it is within someone, it grows and festers like a tainted wound. It is possible though, in very rare situations, for the effects of corruption to be lessened. This is always an extraordinary occurrence, and might happen once in a campaign at most. It could represent a [...] powerful and singular event. This can only reduce Corruption by a small measure, usually no more than 1 or 2 points, though the GM can modify this depending on the nature of the event."

A shadow of suspicion by the Apothecarion or DW superiors could work as a narrative tool to add some depth, especially by playing up how an Apothecary from a different Chapter might perceive an anomaly with the PC’s gene-seed (that is, it's difficult to establish for a relative outsider whether it's flat-out corrupted, or just within the limit of acceptable drift for the PC’s Chapter), just like Lynata said, but I find it difficult to imagine how such a storyline could be brought to a close in a satisfactory manner - earning the permanent suspicion of a sizeable chunk of DW personnel, the inevitable conclusion of any such event, even after a redemption character arc has played out, is more heavy-handed than what I had in mind, and should be introduced with care and after some deliberation.

As for other possible plot hooks, in the case of a Techmarine, my most corrupt player thanks to some careless interfacing with tainted machine spirits, this could lead to a greater drive to replace unclean flesh, but if you can think of something other minor roleplaying or narrative effect, I'd love to hear it.

Ultimately though now I think I will leave matters partly in the hands of my Apothecary to see whether he wants to go ahead with a bit of a friction with teammates, and in parallel use xeno contamination and an NPC Apothecary to scare the players by serious doctor-faces and mumbling, just to declare them clean after the desired RP effect has been reached

*Edit: Also, if I were to play the actual xenos contamination straight, what would be the best species for that? No immediate takeover (mental or physical), just gradual turning.

But since you were talking about Apothecaries, the surgical extraction of tainted/mutated flesh could be something for the player to consider, too.

This is a tricky one; I don't think I've heard of such a species yet , but I'm sure it would be easy to make something up. There's a lot of inspiration to be found in various creatures or even plants (like Brainleaf) that take over their victims immediately; all you'd have to do would be to drag out the process for either an offshoot or an entirely new species.

Some ideas off the top of my head ...

• An alien plant sends out spores which can be inhaled. Once in the body, they trigger an infection within the brain that slowly drains the victim's enthusiasm (Toughness Test once per hour, on failure gain -5 WP characteristics damage), to a point where they would just sit down and die of neglect. Will this defense mechanism intended to protect the native flora against intelligent plant eaters threaten the Kill-Team? (inspired by a mixture of The Happening and Serenity )
  
  To deal with the Marines usually wearing sealed helmets, you could have the spores appear out of nowhere as a cloud of thick flakes borne on a soft wind. Some of the flakes settle on the Marines' visors, and an attempt to brush them away with their fingers leaves a foggy smear reducing Perception, Ballistic Skill and Weapon Skill by 20. Better hope the Apothecary is clever enough to notice the likely cause of his Battle Brothers' lethargy!

• Another Kill-Team invites the Battle Brothers to a feast to celebrate a recent mission. The Kill Team's leader, a Space Wolf, presents the players with an animal native to the world they visited last, and which he had hunted and slain by himself, befitting Chapter tradition. Unbeknownst to the Astartes, the animal was infected with the larvae of mindburrow worms that begin to hatch after the flesh is consumed. The worms will slowly travel through the host body alongside the spine until reaching the brain and begin to alter the host's thinking.
  
  After the members of the other Kill-Team had already started to behave erratically, the infestation fully comes to light during combat drills aboard the Watch Station, when one of the Marines goes into a frenzy and starts to attack the player characters. The infected Marine has to be put down, but whilst his corpse is quickly taken to the laboratorium for dissection, the remaining members of his team, reacting to their comrade's demise, quickly worsen in their condition and become paranoid, attempting to flee the Watch Station. Now the players must devise a plan to stop their wayward brethren from escaping or sabotaging the station, whilst at the same time being at risk of the very same fate.
  
  Will the autopsy reveal the cause of the dead Marine's frenzy in time? What will the player characters do once they know what is inside them? Can the Apothecary safely extract these dangerous creatures before it's too late? And .. who can they really trust?

• Another Kill-Team invites the Battle Brothers to a feast to celebrate a recent mission. The Kill Team's leader, a Space Wolf, presents the players with an animal native to the world they visited last, and which he had hunted and slain by himself, befitting Chapter tradition. Unbeknownst to the Astartes, the animal was infected with the larvae of mindburrow worms that begin to hatch after the flesh is consumed. The worms will slowly travel through the host body alongside the spine until reaching the brain and begin to alter the host's thinking.

  After the members of the other Kill-Team had already started to behave erratically, the infestation fully comes to light during combat drills aboard the Watch Station, when one of the Marines goes into a frenzy and starts to attack the player characters. The infected Marine has to be put down, but whilst his corpse is quickly taken to the laboratorium for dissection, the remaining members of his team, reacting to their comrade's demise, quickly worsen in their condition and become paranoid, attempting to flee the Watch Station. Now the players must devise a plan to stop their wayward brethren from escaping or sabotaging the station, whilst at the same time being at risk of the very same fate.

  Will the autopsy reveal the cause of the dead Marine's frenzy in time? What will the player characters do once they know what is inside them? Can the Apothecary safely extract these dangerous creatures before it's too late? And .. who can they really trust?

I like this plot hook a lot. It could be a resupply mission on a minor Watch Station far from Erioch, so 1) the isolated setting is begging for a nice survival horror run and 2) the Kill-Team have to protect the only Apothecary present. It's a neat one-off mission, thanks, I'll be using these ideas. The other one is also promising, but forcing the removal of helmets that way is a bit heavy-handed, and my (genre-savvy) group would quickly put me off tracks, so I need to tweak that a bit so that the players won't scream railroad. I do like the plant ruining their day though.

The reason I'm thinking along the lines of gradual infection is that immediate takeover, like Genestealer* or Bruul Parasite infection, can't be done to player characters easily - it requires a special type of mature roleplayer to enjoy slowly spiralling his very own special snowflake down toward death and destruction without giving himself away, and NPC Marines getting the infection wouldn't have a very strong impact on players exactly because of this subconscious metagaming reason: my group would find it inconceivable to consider something like this as a real threat to them.

*I'm aware that the Purestrain ability Genestealer's Kiss in MotX explicitly states the Marines are immune to this (why though? makes no sense), but I recently listened to a BL audiobook, Incorruptible by David Annandale , where the grand plot twist ( obligatory SPOILER ALERT ) is that a Grey Knight Purifier's being corrupted - just not spiritually by Chaos, but genetically by a Genestealer. The only concession to him as a SM is that he's able to regain control of his faculties for a limited time with extreme effort when the brood's telepathy field is weakened. Thing is, it looks like it can be reasonably done.

The Corruption ideas you float are quite interesting: it would be fun to mess around creating a system which deals with with different sources and thresholds of corruption; it's actually surprising no one did it before. It sounds like something detail-obsessed 40k autists would like to dabble with (not that I'm not one of them occasionally) . I have a major RL project ongoing, but I might just revisit the idea when I'm done with that - though it's probably hard to create such a system without multiple unwieldy tables, and, uh, a lot of time. Meanwhile there's a sort of half-baked houserule I found which proposes a no-set-Req system with adding Corruption Points for excessive Requisitioning as a balancing factor - this could be nicely turned around as docked Requisition or a Renown level penalty during arming could symbolise the diminished trust the character suffers from. Of course I'd reserve such measures for players in the high Corruption range, but still, it's something.