The Invitation (Tales from the Dark Millenium)

There is a mass murderer on Cauldrus. Hired by the planet's governor to find the killer and end their slaughter-spree, headhunter Lowen Tegget travels to the long-dead city of Namogrod. But when Canoness Olga Kamaranz of the Adepta Sororitas arrives, a coterie of Battle Sisters in tow, Tegget realises that this hunt will be the most difficult he has ever performed. Read it because
It's a tale by Dan Abnett that explores the seedy underbelly of the Imperium from a new perspective, and to surprising results…