'15th Hesperan Lancers,' he murmured. 'Assigned to Horus's 53rd Expeditionary Fleet. It looks like some of the rebels stayed behind when their ground forces left the planet. They must have been meeting with that traitor Archoi and arranging delivery of the machines when we arrived.'

'And they've been biding their time ever since, waiting for the right opportunity,' Kohl snarled. 'That damned magos has embedded his warriors into every one of our combat units. We've got to warn the primarch or we may well have a massacre on our hands!'

At just that moment, Brother Vardus leaned out from the stairwell entrance. 'Movement on the stairs!' he hissed, 'coming from above and below.'

Kohl stared back at Vardus. 'Above and below simultaneously?'

Vardus nodded. 'They're moving quietly. Might be a pair of patrols.'

Suddenly, Askelon pointed across the cavernous space. 'I can see movement on the opposite gantry-way!' he said quietly. 'They're carrying something.'

Nemiel felt the hairs on the back of his neck go up. He looked down at the assembly room floor. Magos Archoi was standing there, surrounded by a circle of bemused rebels. The traitor's hooded head was tilted upwards, looking directly at him.

'They know we're here!' he cried, drawing his crozius from his belt. 'It's a trap!'

Lasgun fire erupted from the gantry-way on the opposite side of the building; red bolts hissed through the air, gouging craters from the permacrete wall in a string of sharp thunderclaps. A heavy bolter began to hammer away, spitting tracers across the intervening space in a series of measured bursts. Rounds struck many of the hanging chains, splitting their links and dumping their grisly cargo to the ground.

Nemiel fired a burst in the direction of the heavy bolter and activated his vox-bead. 'Invincible Reason, this is Brother Nemiel!' he cried. 'Can you read me?' He was answered with a rising screech of static. The Redemptor went through a score of frequencies and got the same result. Archoi's traitors were jamming the vox-channels.

Fire erupted from the stairwell behind Nemiel. Autoguns clattered and lasguns spat bursts of light at Marthes and Vardus, who responded with a brace of fragmentation grenades. Marthes levelled his meltagun down the stairs and fired a howling blast, then ducked out onto the gangway. 'There's a platoon of skitarii coming up the stairs!' he shouted.

Dark figures were rushing at them along the gangway from the far side of the building, firing bursts of lasgun fire as they advanced. Kohl and Ephrial exchanged fire with them, dropping several with well-aimed shots. A burst of heavy bolter fire answered them, stitching the two Astartes with a stream of shells. Both warriors staggered beneath the hits, but their armour turned aside the blows.

'Marthes! Put a shot on that gangway!' Nemiel yelled as he leaned over the thin metal rail and levelled his pistol at Magos Archoi. The traitor didn't even flinch as the Redemptor laid his aiming point at the centre of the darkness beneath his hood and let off a burst. The shells flew straight and true - and detonated harmlessly against a force field just a few scant centimetres from their target. The officers with the magos drew laspistols and returned fire, striking Nemiel once in the leg and abdomen.

Marthes shouldered his way onto the catwalk and fired his meltagun at the distant heavy bolter. The microwave burst struck the weapon and the gangway beneath it and superheated the metal in a split second, vaporising them in a fierce concussion and hurling burning skitarii to the assembly floor below.

'We're cut off!' Kohl shouted as he picked off another of the charging skitarii. 'Where do we go from here?'

Nemiel glared down at Archoi. Several metres away, one of the burning skitarii had become entangled in one of the chains on the way down, and now he thrashed and twisted in the air as the flames consumed him. On impulse, he bolstered his pistol. 'Follow me!' he said, then put a foot onto the rail and leapt into space.

The thin metal of the railing bent beneath his full weight, throwing him off balance, but his leap carried him far enough to reach one of the grisly, corpse-strewn chains. He grabbed hold with one hand and slid partway down its length before the slippery metal snaked out of his hands. Nemiel fell the remaining few metres and landed atop the lead siege gun. A tech-adept rose up beside him, raising a crackling arc-torch, but he may as well have been moving in slow motion. The Astartes smashed the traitor aside with a sweep of his crozius and began to run along the downward-sloping hull towards Archoi and the rebel officers.

'For the Lion!' he roared, raising the crozius aquilum high as he launched himself at the traitors.

General Morten shifted uncomfortably in the shuttle's oversized jump seat and tried to conceal the scowl on his face by pretending to study the view beyond the small window at his left. 'If I could perhaps get some idea of what it is you're looking for, I could arrange for a presentation from the garrison's senior officers.'

'That would defeat the purpose of the inspection,' Zahariel replied from his seat across the shuttle's passenger cabin. 'In fact, it would be best if the troops never knew I was there.'

'Very well,' Morten rasped, though Zahariel could see that his weathered face was still troubled. The Terran officer stared out the window for a moment more, debating what to say next. After a moment, he drew a deep breath and said, 'You asked me to inspect the troops at Northwilds to provide a cover for your own activities.'

'That's right,' Zahariel admitted. He didn't want to lie to the man any more than he had to. 'We'll part ways once the shuttle lands, and it's likely I won't be returning with you back to Aldurukh.' He spread his hands. 'I regret that I can't be any more candid, but this is Legion business. I'm sure you understand.'

'Yes, of course,' Morten said readily, but there was no mistaking the wary look in the old general's eye. For a brief moment, Zahariel wondered if there was something that the general was hiding but he quickly dismissed the thought with a flash of irritation. He had no reason to distrust Morten, Zahariel reminded himself forcibly. The man was, by all accounts, an honourable and dedicated soldier, and had every reason to wonder at Zahariel's request for an unannounced inspection of the garrison at the Northwilds arcology. The fact was, Zahariel couldn't afford to make his presence known to the local troops or the Administratum officials struggling to maintain order across the arcology's war-torn sectors; it would lead to pointed questions that he could ill afford to answer.

The last thing he wanted was for General Morten - or worse, Magos Bosk - to learn that a member of the Legion was meeting secretly with rebel leaders in the midst of the most hotly-contested population centre on the planet. It was unlikely that either of the Terrans would take the news well. As much as he hated the idea of concealing his actions, Zahariel was forced to admit that, when it came down to it, Morten and Bosk acted in the best interests of the Imperium, not Caliban itself.

Shafts of late afternoon sunlight slanted through the window to Zahariel's right as the military shuttle began a wide, diving turn towards their destination. The Librarian craned his neck to peer out the window to the northeast, where the arcology rose sharp-edged against the backdrop of the weathered mountain range further north.

The Northwilds arcology had been built according to the standard Imperial template; it was an irregularly-stepped pyramid that, even still in its initial stages, was five kilometres wide at its base and rose more than three kilometres into the cloudy sky. Narrow streets radiated away from the arcology across the plain, surrounded by hundreds of smaller buildings that had yet to be subsumed by the structure's ever-expanding footprint.