http://wh40k.lexicanum.com/wiki/Obliterator_Virus

The Obliterator virus is a chaos pathogen that fuses those infected with technology. This would be perfect for putting in a 'Borg'-like encounter in Rogue Trader. The scenario would begin with discovering a space hulk; one that looks like an amalgam of demon flesh and technology. Obliterators would then teleport onto the players' ship and begin infecting (assimilating) members of the crew.

Here's an example:

Obliterator Cultist

WS 30 BS 35 S 40 (8) T 40 (8) Ag 20 Int 40 Per 30 WP 20 Fel 20

Movement 2/4/8/12 Wounds: 17

Skills: Tech-Use(Int); The cultist will access ship systems when possible to disrupt its enemy, such as turning off life support or venting sections to space.

Talents: Two-Weapon Wielder, Plus any Weapon Training.

Traits: Armor Plated, Auto-stabalized, Darksight, Machine(5), Natural Weapon(servo-fists), Sturdy, Unnatural Strength and Toughness(x2)

Armor (Machine): All 6.

Weapons: The cultist can manifest a variety of weapons to suit the situation and never runs out of ammunition. It may manifest a new weapon as a move action each round. Any character grappled by the cultist must make a challenging toughness test or contract the Obliterator virus.

Gear: Internal Vox and combi-tool, Personal Teleporter, systems sealed to void, inbuilt mag-boots.

With unnatural strength, toughness and armour plating the cultists are going to be nasty customers.

You don't seem to have definied what weapons the cultists can use. Perhaps limiting them to pistol and basic las, flamer and plasma weapons since they weren't full blown tech-marines or terminators to begin with.

Here's the Obliterator I used when my players found it wise to assault a heretek-fortress:

WS 40 BS 55 S 80* T 50* AG 20 Int 35 Per 35 WP 45

Wounds 20. Armour 9. Hulking size.

Heavy flamer, plasma cannon, powerfist, multi-melta, heavy bolter. Choose one every turn.

The * means it is Daemonic in origin, and it is fair to say the PCs gain 1 corruption from the mere sight of this monster.

Didn't bother using more rules than that on it. This one was made with daemonic chaos-worshipping heretek.

I would suggest that the obliterator virus only starts out with ressources at hand. And an unmodified human isn´t a good starting ressource, aside from minor augmetics and a personal weapon, there isn´t much tech to convert at first.

Freshly infected may be able to pass as humans, concealing assimilated weaponry, able to "pop out" their guns at any moment.

As they assimilate armor, technology and weapons, the obliterator may not be able to pass as a human and hide till he killed enough victims. With the assimilated gear he will grow and eventually transform into a full blown obliterator that can create any weaponry he needs.

This would make things more interesting and also dependent on the environment. Obliterator viruses may have a hard standing on feudal worlds while thriving on hive worlds and being a monstrous threat on forge worlds.

What do you mean by "Obliterator Cult"?

Do you mean as presented in the latest CSM codex, or do you mean to infer that Obliterators may also be made from common humans or tech priests?

By the latest version of canon, the Obliterator is a Heretek Tech Marine. They make a great adversary for either RT or DH as they are in pursuit of Archaeotech, Heretek, Warp-tek, etc.

A sick stat line is also in order, as they are basically a tough Tech Marine Terminator on warp-juice. Unnatural Toughness, Unnatural Strenght, psycho armor, refractor fields, etc are all on deck.

I wouldn't limit the weapons they can manifest other than to limit it to energy/flame and some level of man portable (up to heavy, but not vehicle only). While they may not be limited on ammo per se, perhaps they should be forced to recharge. Variously, any heavy weapon could be considered an Overheats representing the potential damage done by them pushing themselves.

In short, a mere one standard Obliterator is probably enough for the characters to handle by itself. I would also assume that they can manifest any weapon with an Energy damage type. In fact, if you take the Heretek Tech Marine route, a single Obliterator is likely a good candidate as a campaign level nemesis.

The virus is now reduced to an Imperial "theory". In fact, they may just as well be demon possessed. Or, fused with a demon. Or, whatever you want.

Now, an Obliterator 'cult' that was seeding mundane humans with their own virus/possession/whatever does indeed make a great adversary too. I would say the 'normal' humans would be on the level with combat servitors. Perhaps, with less mastery, they can only use one ranged weapon type, or it takes longer to manifest.

In short, I was thinking Obliterators for adversaries too

I confess that in the last game of DH I ran, I threw in a tree infected with the Obliterator Virus... If you're going to use obliterators, which are basically just people infected with something that is one part virus, one part nano-bot and one part daemon, there's no reason to NOT use your imagination - if it is a living creature, it's ripe for infection! Servitors, cyber-mastiffs, tyranid weapon-beasts, plants... they're all legitimate targets.

Hm. That gives me an idea... an obliterator swamp. An obliterator-host dies in the midst of a swampland and infects all the life around it. Small, semi-robotic mosquitoes buzz around trees that drape with semi-organic network cables and void-shield emitters. Segmented steel fish splash around in the water as larger predators lurk under rusted metal foliage, hunting by inbuilt radar. Everything guided by a single, malign entity that seeks only to increase the size of the swamplands, possibly at the risk of nearby imperail settlements. Perhaps the nearby settlement trades with the swamp, offering it newborn children and the greatest part of their harvests in order to stay the daemon swamp-mind's wrath. Perhaps still, those people harvest from the swamp, gathering the solar gathering leaf-panels and the plasma-emitting hearts of the local wildlife for trade to those that are unknowing of the settlements terrible secret...

Sure, more Heresy than Trader, but it's still something that could be fun!

Obliterator cult are alos from corrupted Adpetus Machanicus priest. See the Dark Apostle books.