How does this backstory sound to you, DM?
[ +- ] Antonius Orastant: Feared Spymaster of the Unknowable Double-Helix
Inquisitor Killain had heard of the exploits and actions of this incognito agent of the Logis Strategos, who doubles as an agent of the Officio Medicae. He heard that this person was working on a plague on a pandemic scale on this particular planet, and he discovered some strange political factions in the local governments on that planet channeling Chaos into their planet in order to exert control over the populace, and in so doing, their power plays affected the tithes sent back to the Imperium.
This fabled agent, somehow, managed to weaponize the plague, and used "strategic placement" of the virus to bring the populace against the magistrates of the local government to the whims of the populace. Though there is no paper trail other than the redacted espionage reports in the Adeptus Administratum, this agent was somehow not only able to bring the local government under tighter control, but he was able to do that in a way that, in the micro level, it seemed as though it was a natural occurrence, rather than an orchestrated series of events.
There were about 500 different Officio Medicae agents on that particular planetary system at that time, helping curb the pandemic, Killain was not able to ascertain who that agent was, until days after receiving his inquisition assignment, an unaddressed message came to him from an agent of the Officio Medicae, an Antonius Orastant of the Order of the Unknowable Double-Helix, asking if he could be assigned under this inquisitor's squad for strategic use.