Papa Gallo slapped Snowdog hard in the face.

'You owe these people, Stanker. You took their money and possessions in exchange for safety.'

'What?' snapped Snowdog, pulling his arm free of the old man's grip. He pointed to the sky and said, 'Hey, I gave 'em a place to stay out of the cold and kept these damned things from killing them. I think I done my share. I got problems of my own now.'

Tigerlily moved up to stand behind him and nudged him in the ribs, but Snowdog ignored her, too intent on the confrontation with the old man and the wounded Silver.

'I don't think so,' said Papa Gallo, folding his arms.

'Tough,' retorted Snowdog, 'Anyway, all the stuff they gave trie's gone up in smoke.'

'Not our problem. You owe us.'

Tigerlily nudged him again and this time he shot her an irritated glance. She nodded in the direction of the blazing warehouse. He followed her gaze and felt a hot thrill of fear slide around his body. Hundreds of soot-stained civilians, gathered silhouetted in the flames, many of them armed. Armed with weapons Snowdog himself had given them.

They were on edge and looked ready to use them.

Snowdog locked eyes with Papa Gallo and saw the fierce determination there.

He saw Jonny slide a shell for his rifle from the bandoliers and shook his head.

'Okay, man, you win,' said Snowdog, kneeling beside the unconscious Silver. 'What do you want? But be quick.'

'There's a lot of wounded here and you don't have the supplies to deal with them any more.'

'And?'

'And we need to get these people some help. I want you to lead them to the nearest medicae facility,' stated Papa Gallo.

'Shit, man, the nearest one still standing's in District Quintus' protested Snowdog.

'Not my problem,' repeated Papa Gallo, and as Snowdog looked at the bleeding girl beside him and the many weapons lacing him, he realised he had no choice.

'Okay then,' he shrugged, shucking his backpack onto his shoulders and gathering up Silver in his arms. 'Let's get gone. You don't wanna be hanging around with those things flying overhead.'

The lictor thrashed against its restraints, flesh hooks lashing out at the armoured glass that separated it from those who observed it. Bound to three upright dissection tables shed together, its powerful muscles bunched as it attempted to break free, but the restraints rendered it immobile. Even so, it had killed two magos-biologis who had wisely failed to observe full xeno-containment procedures and wounded a third who had subsequently been put to death for his lapse.

With the lictor's capture, Magos Locard's work had professed with a new urgency following the failed attempt to destroy both hive ships between the defence lasers and the Imperial fleet. Things had gone from bad to worse when the cowardly Simon van Gelder had attempted to flee Tarsis Ultra and treacherously shut down the valley's defences.

The aerial exclusion zone had eventually been re-established, but not before hundreds of gargoyles and their monstrous brood-mothers had penetrated deep into the valley of Erebus. It appeared that they were without the controlling influence of the hive mind, as the majority of the creatures had reverted to their basic, animalistic instincts, nesting in the caves of the valley sides and attacking small groups of civilians. Others had rampaged through the densely-populated quarters of the city, killing in ah orgy of random violence for two days before being hunted down by volunteer groups from the Erebus Defence Legion.

The fighting at the District Quintus wall raged with undiminished ferocity, the tyranid swarm almost doubling in size with the addition of yet more creatures as they were drawn to Erebus by the single remaining hive ship. Time was running out for the defenders of Tarsis Ultra and Magos Locard was their last, best hope.

Deep in one of the Adeptus Mechanicus vivisectoria, Magos Locard held forth to an assembled audience of Colonel Stagler, Major Satria, Lord Inquisitor Kryptman, Chaplain Astador and Uriel. A blank-faced servitor with augmented bionics grafted to its head and upper body stood in attendance to the magos, carrying a silver pistol case. They watched the lictor through the armoured glass with revulsion, its physiology repugnant, its mental processes beyond their comprehension.

'As you can observe,' began Locard, 'the lictor organism, even restrained by level three xeno-containment - unfortunately the highest level available in these facilities - is still 45.43% lethal.'

'So why are you keeping the damned thing alive?' demanded Stagler. 'Why not just kill it?'

'To defeat these aliens, we must first understand them,' explained Kryptman. 'When fighting the ork, the hrud, the galthites, the lacrymole we do so armed with knowledge of their undoing. To fight one tyranid is not to know another. Their adaptive nature is what makes them such superlative predators. It is their greatest asset and, potentially in this case, the one weakness we might exploit.'

'In what way?' asked Uriel.

'Tell me, Captain Ventris, have you heard the phrase "to turn an enemy's strength against him"?'

'Of course.'

'That is exactly what we intend,' said Kryptman with a sly smile. 'Magos Locard, if you please.'

Locard nodded and turned to the servitor, his mechadendrites unlocking the pistol case with precise turns of cog-toothed keys that slid from their tooled digits. He lifted a magnificently crafted silver pistol and a large calibre glassy bullet from the foam interior. With exaggerated care he slid the bullet into the breech and handed the weapon to the servitor as his mechadendrites relieved it of the case. At a nod from Kryptman, he spun the locking wheel that led into the lictor's cell and said, 'Proceed with instruction one.'

The servitor turned and pushed open the heavy door, marching to stand beside the dissection tables. Locard sealed the door as the lictor renewed its efforts to break free. The servitor approached and raised the pistol, pressing it against the fleshy portion of the lictor's midsection.

'What in the name of the Emperor is it doing?' asked Uriel.

'Observe,' said Locard, with more than a hint of pride in his voice. He pressed a thumb to the intercom and said, 'Perform instruction two.'

The servitor pulled the trigger, firing the glassy shell into the lictor. Ichor spilled from the wound, hissing on the vivisectoria's floor. Without pausing, the servitor placed the pistol carefully on the floor as Locard released the dissection table restraints.

In a blur of motion the lictor pounced, its severed upper claws smashing the servitor across the room. Its heavily augmented body cracked the glass, drawing cries of alarm from the observers.

Uriel and Astador unholstered their bolt pistols and aimed them through the glass.

'Wait!' cried Kryptman.

The lictor charged the servitor, its lower arms tearing into its grey flesh in a frenzy of violence. Blood sprayed the walls as the beast ripped its victim to shreds, tearing and gouging its body until mere was nothing even remotely humanoid remaining. The beast reared up and hammered against the glass. Fresh cracks spread wider, rapidly spiderwebbing across its surface.

'Kill it! Kill it!' shouted Colonel Stagler.

Before Uriel and Astador could fire, the lictor doubled up, dropping to the floor of its cell. The beast let out a keening wail, its entire body convulsing as a frenzy of rippling motion undulated within its flesh.

'Ah yes, now it begins,' noted Locard. 'Resilient, but I expected that, what with its genome being relatively fixed.'

'What's happening to it?' said Uriel, staring in disgust at the convulsing monster.

The lictor fell onto its back, wracked by massive spasms, its body heaving into a giant inverted ''U''. Even through the glass, Uriel heard a loud crack as its spine snapped. The lictor's flesh split and monstrous growths erupted from within, its flesh writhing in uncontrolled evolution. Semi-formed limbs writhed from its viscera and other unnameable organs swelled from its mutating body.