Okay, my next adventure will introduce the Acolytes to a small Imperial Death Cult working inside Scintilla Hive Sibellus. The idea is (roughly) this:
The Cult is an Imperial Death Cult that worships "Emperor Revenant" or "The Lord of Death". Their core-belief is that Emperor has died, self-resurrected himself and essentially conquered death itself. Some of them actually belive Emperor has control over life and death and their victims only die because Emperor allows it. The core group of the cult is hired from an underhive gang which has a few common themes with House Escher of Necromunda. The highest ranking cultists are sometimes hired as Death Cult Assassins by several local and Imperial organizations.
Now, what I would want is some ideas on rituals, symbols, clothing and day-to-day affairs that would fit this Cult. Oh, and these girls do not have any chaos involvement, they are just a bit mislead and harbor beliefs that some hard-liner might consider wee bit heretical. They are part of the setting, so to speak, not the actual focus of the investigations... The actual focus of investigations is another seemingly pro-imperial cult that believes Emperor is sending messages to them on psychic radiowaves...
. Giving them masks is never wrong. How about about very sleek mask that do not emulate any facial features? This could underline the point that they are just tools of the Emporers Will in that moment.
Their secret means of communication is Imperial Guard battle language? I would have thought they would develop their own, somewhat more exclusive code language. Sooner or later they WILL be found out when any Sergeant can freely translate their codes.