"The fact that the Ultramarines have not come here to the Reach is simply a matter of practicality," Inquisitor Jiang explains, his manner still very matter-of-fact. "Aboard a Rogue Trading vessel geared for perilous transit through the Warp, with an expert Navigator hand-picked for the task, I myself had a very difficult time making it through the fierce storms in the Immaterium to reach this sector of space from Ultramar. To risk a strike cruiser of Ultramarines on the same transit would be foolhardy, and if they wished to come safely through the Jericho-Maw Warp Gate they would have to cross the entire galaxy first."

Again the Inquisitor's lips twitch into a thin smile.

"That is not, of course, to say that they stand idle. They are pursuing the traders' activities and connections on the Ultramar side of the storms quite vigorously. But it falls to us to match their fervor on this side of the investigation if we are to eliminate this threat once and for all."

He reaches into his robes, pulls out a dataslate embedded in a thick leather cover, and begins scrolling through his files.

"I have already described the weapons' flaying effect, but to help you render effective medical treatment, Priest, you would be better served by pict-captures. To give you those I will need to access my data-safe archive, which I've left in my quarters. So I can't get that to you now, but I'll be sure to make all that I have available to you before we arrive at Zanatov's Harbour. I'm afraid it isn't much to go on, but it should be better than going in blind."

You may note that the Inquisitor said 'Priest' rather than 'Apothecary,' just as he called Jaden 'Stormseer' rather than 'Librarian.' It seems he's done his homework on your kill-team.

Spoiler: OOC
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Practically speaking, the information which Inquisitor Jiang will provide Rafen (probably offscreen) will be enough to make sure he definitely faces no specific penalty in treating gauss weapon wounds, but won't be enough to give any kind of bonus, either.