‘Attackers!’ Varren’s battle roar shattered Ison’s reverie, and the other legionary opened fire on the mad horde of bloated figures that came falling through the hatches and into the corridor before them.

It was Helig Gallor, with typical Death Guard aplomb, who had named these transformed foot-soldiers ‘the flyblown’. Humans bitten and transfigured by exposure to the mutagenic venom of infernal pests, they were mindless killing machines that lived only to destroy or to build their numbers by infecting others. In a closed environment like the Walking City, they were akin to a fast-acting virus. If not burned out, that infection could spread unchecked.

There was no cure, only excision. Varren’s volkite guns shrieked white fire and blasted the front rank of the flyblown into ashen gobs of matter, but their fellows stamped their remains into the deck and stumbled on.

The creatures were ugly and misshapen. Something of the people they had once been could be determined by their clothing and the tone of their skin – both now stretched to the point of splitting by the swelling of diseased tissues beneath – but no features were recognisable. Burst human eyes became insectile, turning into grotesque, multifaceted jewels. Tiny flickering wings and mandible parts emerged from boils on bare flesh, quivering and rasping as the mutation took hold.

Ison’s bolter barked, the concussion of the shot deafening many of the civilians crowding behind him. He could feel their terror churning like boiling water. The passageway beyond was a dead end, and if the Knights-Errant fell against the mass of the flyblown, the humans would be fodder for the mutants.

The mass-reactive rounds from his gun blasted through the creatures, but each one he killed erupted with quivering new life as buzzing streams of birthing flies swarmed out to sully the air. Varren’s ray weapons turned everything they touched to powder, but even he was in danger of being overwhelmed.

‘Can’t hold these off much longer,’ snarled the other warrior. ‘Stop pissing around like a tyro and do what you have to!’

The psychic hood at the back of Ison’s head glowed at the unvoiced thought in his mind. ‘If I use it here, the civilians will be at risk.’

‘Forget them!’ Varren retorted. ‘Just do it!’

Damn him for being right. Ison’s jaw set as he let his bolter hang free on its sling, and he brought his hands together in a prayer-like gesture. Tiny jolts of crimson-white fire collected in the crystal matrix of the hood, growing to shimmer down his arms, gathering around his fingers. The flyblown were almost upon them now, and Varren’s pistols were sizzling with built-up heat.