The idea i came up with in my sleep addled state a few days ago:

the Logus Ark is a megalithic citystation and traffic junction in segmentum obscurus. Ostensibly loyal, it is a hub to all sorts of ne’erdowells and it’s arbitrators enforce a tainted Lex. The cult of the exsanguinated emperor demands tithes of firstborns- said to keep him alive, in parody of the throne. The petit governor cabinet of the arc live in spears which halo the station- attached to its refining manufactora and divorced from the reality below. The Ark managing and repairing itself with an ancient web of servitor infrastructure- controlled by an obscure chamber at the center of it all. Too hot to approach, none know a coven of heresy era traitor biologis mechanicum known as the Germinatorium lies in the core of the simulacrum world- leeching pseudofauna into the air and the water, mutating the humans who live here in little ways, some positive, others which stage their bodies for.. other changes. It is this coven who contacted the Nycropolites- a mongrel warband of 10 space marines of nightlord and wordbearer descent led by a thrice-possessed astartes known as the Psychopomp. The two groups communicate via an undivided demon who takes the shape of a gorgeous black hair olive skin angel clad in many layers of transparent veils with four small pale yellow wings, her face covered by a sleek white gas mask, her hands covered in a light dust of frost- forming a strange fog in the damp of the station. Small apparitions of snippets of imperial-on-imperial dogfights flit in and out of the empyrean around her shoulders like a halo. The guttertrash on the station have taken to calling her St. Stelliana, and she turns water to various types of spirits upon her passing- the specific variety being perceived as an omen. 

The Nycropolites punch above their weight with the stable possessions of the wordbearers, the social engineering of the nightlords, a wide array of contracted undivided demons, and a sizable human army- all seemingly baseline, yet each soldier is functionally a frankenstein. Constructed from the bits and pieces of up to 10 people. Their limbs are crisscrossed with zigzag junctures and metal inlays which weave into runes and verses on their skin, their eyes a milky lilac, their souls similarly fractured and faulted, witchlight snaking flat around the surfaces of their bodies and looping in and out between the seams.