The basic concept revolves around the short blurb written by Gav Thorpe in his Thorian Sourcebook made for the Inquisitor game by Games workshop.
Since the Ordo Sepulturum is so specialized, I figured what better way to hunt zombies than haz-mat containment suits which were originally made to enter plasma core generators worn by the Ordo's Chamber Militant? Add in a few robots that can not be mutated by the Plague Virus as forward scouting troops as well as a few other fluffy sundries, and I think its an army with quite a bit of originality as far as armies commonly seen on tabletops.
The Ordo Sepulturum gathers members for its Chamber Militant from numerous sources, as most Ordos do. Predominantly from areas of Thorian sympathizers such as the Adepta Sororitas 'Order of the Argent Shroud' Convent Prioris stationed on Terra as well as the ancient Adeptus Astartes Praeses 'Night Watch' Chapter whose express purpose is to crusade near the Eye of Terror. Guardsmen of Cadia are also requisitioned, but on a more infrequent basis so as to keep the spread of the Zombie Plague to a minimum due to their susceptibility of contracting the Plague Virus.
The Ordo has recently retained various mechanical devices made by House Krast of the Adeptus Mechanicus stationed on the Forgeworld of Sabbatorus in the Chinchare sub-sector. The devices are quite powerful and are also quite rare. The Machine Spirit that dwells within each of these devices is quite unlike those found in other Imperial technologies that can be found elsewhere. The Ordo felt it was getting its due, but the Adeptus Mechanicus has an underlying agenda other than bowing to the will of the Holy Inquisition. House Krast hopes to retrieve samples of the Obliterator Virus to study outside of the watchful eyes of the Inquisition. The rumored bio-mechanical properties of the Obliterator virus is of much value to to House Krast, who view possessing such knowledge of its workings as a certain political leverage over other Houses on Mars.
The 'Night Watch' Chapter has been severely depleted by Abbadon's 13th crusade who were at one time the most abundant Adeptus Astartes chapter present at the campaign. They have learned the lesson taught during that black period in their history and with the help of the Ordo, they have acquired enough suits of tactical dreadnought armor to make them second only to the Dark Angel's Deathwing company. This pragmatic approach has exponentially raised the lifespan of each Brother marine far beyond previous expectations. The Night Watch have made a pact with the Ordo Sepulturum to be their standing, rapid response force because the need is so great to eliminate the Plague threat near the Eye of Terror. The teleportation homing signals installed in each of House Krast's archaeotech devices are so powerful that the remnants of the 'Night Watch' are able to key into their position with great precision from nearly two parsecs distance when the Terminator's transmission are amplified by their battlebarge's transponder.
"In the Name and Honor of His Divine Majesty, The God-Emperor of Mankind, I name thee 'Contagion' and commit thee to the Purgation of Righteousness.
May your soul be stripped off all that is malignant and fall below the Valley of Death.
Left to rot forever, to never be free, never return...."
~Attributed to Grand Master Maxmillian Calavera, Holder of the Incandescent Aspergillum and Scourge of the Undead~