Ison closed his eyes and mouthed a mnemonic. In the Angel’s name, I smite thee. A blinding blaze of red lightning leapt from his hands and washed out across the narrow corridor, sweeping up and through the mass of the flyblown. Backwash from the blast writhed around him, buffeting Varren and striking dead a handful of civilians too close to get away – but the psychic weapon did the deed, atomising the mutants in a single wave, turning them and their insect progeny into black dust. As the blast faded, the dark ash settled to the deck in a deep drift.

Varren granted the enemy the only honour they were due, and spat into the mass of their remains. He turned and eyed the young whelp-psyker. ‘You’re good for something,’ admitted the former World Eaters legionary, coming as close as he ever would to anything resembling contrition. ‘I was wrong. You can’t be one of Fulgrim’s – those braggarts are useless.’

He fell silent as he realised that Ison was taking no notice of him, staring away down the corridor.

‘Don’t ignore me, boy.’

Ison shook his head. ‘Something else is coming,’ muttered the psyker. As the warning left his lips, the corridor around them set to a thrumming vibration.

The sound was so bass and low that it made Varren’s teeth hum in his jaw, and around him he saw many of the civilians clasping at their noses and ears as blood trickled from the orifices. He clamped his mouth shut and triggered the gas vents on the breeches of his serpentas, but even as the cool-down cycled he knew he would not have time to dwell.

From around the far corner of the corridor came a colossal, rough-edged shadow that spilled over every surface. Buzzing like chainsword blades, it was a huge, dense swarm of carrion flies. The mass of the insects was great enough to deform metal where it impacted, and it befouled the air with a cancerous corpse-odour.

Only one of Varren’s pistols answered the trigger when he squeezed it, both of the guns still radiating great washes of heat that he could feel through the palms of his gauntlets. The single volkite beam bored through the swarm, killing part of it, but the rest of the mass seethed and parted around the blast.

‘Got another one in you?’ Varren shouted, nodding towards Ison’s psychic hood. The younger warrior was already gathering his preternatural power anew, but the glow was weaker than before and Varren suspected it would not save them a second time.