The black vapors rolled along the floor, the passage of the orb of warpfire stirred up swirls in the deathly fog as it moved closer to the two marines and the new ensuing battle.

Why did it have to be a techmarine? As potential allies were, he wasn't really a suitable one at all.
With regular uninitiated mortals, the processes of technology was a mystical and unknown. For one of the Frater Astrotechnicus however the secrets were known.
Perhaps while not many mastered its many hidden concepts, the techmarines would surely recognize proper mechanical rituals compared to those of the warp. Which meant he couldn't solve any mechanical problems he didn't know with the use of ruinous powers.
Troublesome...

Zuriel watched as one of the mutants hefted a large maul and looked at the Emperors child with a dumb look, with far too many needle like pointy long teeth to properly fit in its mouth. One of the them actually went through its lower lip. Before it bellowed.
With what looked like a gust of wind, the roiling ankle high black fog rolled under the feet of the marines and into another direction. Putting himself so that the marines would be in the way of the mutants and so that he wouldn't end up in the firing line of any other powers manifested by his ork 'friend'.

With what would looked like a tornado beginning from the floor and sucking in all the swirling black smoke, he was reformed, pointing his pistol towards the mutants, the ugly teethed one in particular. And fired.
The beam struck its torso, the poor wretch looked down as the strike didn't burn through his flak vest but instead melted it and it began to drip like molten metal... towards the roof.
The beam danced upwards and hit flesh, the mutant began to scream but was quickly silenced as the flesh began to roll itself out of the way of the purple screaming beam. As the energy beam reached the mouth the mutant was luckily dead, it had been a short final moment.
As the jaws were seemingly bending open it seemed like some sort of wretched warp furie clawed itself into the reality through the gash that was created from the hellish pistol.

Hell pistol, just got a whole other meaning.