Geologists once came from far distant corners of the Imperium to study the cavern arcologies of Calth. Magi from the forges of Mars and the master masons of the Terran Guilds marvelled at their self-sufficiency and remarked often on how seamlessly the artifice of man blended with the vagaries of natural formation.

Horus himself once came to Calth as Guilliman's honoured guest, though no one now remarks on that particular visit. Ingenium Subiaco pauses in his labours and wonders what those magi and masons would make of what has been done beneath Calth's surface now.

A tall man with a permanent stoop that comes from spending endless days bending over highly detailed schemata, the ingenium's craggy face is crowned with a thinning crop of corn-coloured hair. A set of brass-rimmed goggles, complete with noospheric MIU and a full sensorium suite, is clamped to Subiaco's haunted face like some form of surgical device. In the fine tradition of the Ingenium from Calth, he cultivates a long moustache with its ends waxed to points that curl over his florid cheeks.

Long days and restless nights have given him an unkempt look, one at odds with his station as a senior ingenium of the Calth Pioneer Auxilia. A wave of tiredness washes through him and his eyes flutter closed for an instant, but he quickly blinks them open. He has too many nightmares in his sleep to wish for more in his waking hours.

Subiaco stifles a yawn and watches as yet another opening in the bedrock is gradually sealed up. This one is a dead tunnel that delves a thousand metres from a lower branch cavern of Arcology X. The cartographae drones that returned tell him the tunnel is a dead end, but the violence of the war has made accurate readings of the deep caverns next to impossible.

A pulse of thought fades up a noospheric projection of the tunnel's dimensions before his eyes. Subiaco dials down the magnification to view its entirety. The tunnel is five metres wide and curves downwards in a gentle arc for another three hundred metres, twisting through a series of sharp bends before arriving at a water-filled corrasional cave. The deeper reaches of the tunnel are hazed with error-signifiers. Subiaco wishes he had the time and manpower to map them with greater accuracy.

The cavern in which he stands is filled with the tools of the ingenium: blue and grey earth-moving machines, each with dozer blades tens of metres high, bulk crawlers with pneumatic arms capable of lifting a super-heavy battle engine with ease, drilling rigs with conical snouts, and a lone construction engine of the Mechanicum. The noise they make is cacophonous, and but for the aural baffles worked into the mechanism of his goggles, he would long since have been deafened.

Hundreds of men and women of the Pioneer Auxilia are manoeuvring the last of the blast shutters into metres-deep caissons at the mouth of the tunnel, while lumbering tankers of permacrete stand ready. The Pioneers wear heavy, tear-resistant coveralls and bulky respirators, but toil without complaint in the heat, dust and gloom.

They are bent to their labours with pride and determination.

Subiaco understands that pride, it is Ultramarian to the core.

To strive for excellence is the bare minimum expected of Lord Guilliman's people, and to be born in the Five Hundred Worlds is an honour and privilege that must be repaid every day.

The world above is no more, but he and his Pioneers will be builders of the world below.

Subiaco watches the work with bloodshot eyes, but he needs offer no suggestions nor make any corrections. His subordinates know their craft and his instructions are precise, needing no further explanation. Instead, he calls up a fuller rendition of Arcology X, smiling as he realises that Captain Ventanus's hurried marking of a map has effectively renamed this cave complex forever.

Thinking of Ventanus, Subiaco looks up as an Ultramarines sergeant in a battle-damaged suit of power armour approaches him. He does not know this warrior, but the deep blue of his armour is heavily abraded across the breastplate and pauldrons with bullet impacts and blade scars.

Only his helmet is unscathed, painted a fresh crimson that seems oddly fitting.

'Sergeant Ankrion,' says Subiaco, optical filters reading the warrior's name beneath a patina of las-burns on his right shoulder guard. 'Is there something I can do for you today?'

'How long until the tunnel is sealed?' asks the giant.

Ankrion's tone is brusque, but Subiaco understands his urgency. Subiaco calls up a host of data-streams and sifts graphs of work completion sigils with haptic implants in his fingertips.

'The shutters will be in place momentarily. Once the integrity checks are complete, we spray the permacrete and I will implant the locking seal. All things being equal, the tunnel will be secure within the hour.'

Ankrion nods, though he is clearly unhappy with the answer.

'You can't do it quicker?' he asks.

'Not if you want an Ingenium Mark on the work, no.'

'Would more machines speed the process?'

'Of course, but we don't have any more machines,' says Subiaco. 'We're lucky to have the ones we've got.'

'Clarify.'

Subiaco waves a hand at the construction engines and earth-moving machinery, causing his holographic graphs to spin away.

'None of these machines should be here, Sergeant Ankrion. They were all due for orbital transit when the traitors attacked.'

'So why are they here?'

'My understanding is that we have the Word Bearers to thank for that.'

'I'm not in the habit of thanking those bastards for anything,' says Ankrion, and Subiaco hurries to explain himself as the Space Marine exudes a looming threat.

'You misunderstand. The corruption they used to infect the orbital defence systems,' says Subiaco. 'It appears it caused a cumulative arithmetical overflow in the scheduling subroutines of a Defence Auxilia calculus-logi, which saw these engines sit idle on the embarkation platforms while the rest of Calth was being shipped into orbit. Lucky for us, eh?'

Ankrion does not reply, and looks up as the last of the blast shutters is lowered into position with a heavy impact of metal on stone. A squad of riveters move into position, their whining guns securing the shutter in place. Sparks rain down from their work, and the permacrete hoses lift with a hiss of pneumatics.

'This would go quicker if we didn't have to seal off all these dead branches,' observes Subiaco, projecting a holographic representation of the tunnel's structure from the surface of his data-slate. 'For example, this tunnel terminates hundreds of kilometres from the nearest arcology or shelter. There's really no need to expend resources to seal it.'

Ankrion takes a moment to study the gently rotating image.

'Did you find a source for the water in the chamber at the tunnel's end?' he asks.

'No, implying that it is an opening of negligible proportions.'

'In other words, you don't know where the water is coming from?'

'Not as such, but'

'Captain Ventanus's orders are unambiguous,' interrupts Ankrion. 'Any tunnel the termination of which cannot be confirmed absolutely is to be sealed.'

'Sergeant, you need to understand that only a very few of Calth's cave systems are linked. The vast majority spread through the planet's crust in splendid isolation.'

'If Calth is to survive, that's going to have to change,' says Ankrion.

VII.

Shelter CV427/Praxor sits fifteen hundred kilometres to the east of Lanshear, a series of hardened bunkers and armaments storage facilities. It is designed to hold up to a hundred thousand fighting soldiers and a further twenty thousand ancillary staff, together with three battalions of Defence Auxilia personnel.

Its maximum occupancy is listed as one hundred and fifty thousand souls.

In the wake of the XVII Legion's attack it is currently home to over twice that number. Its enlarged caverns and deep constructions are nightmares of overcrowding, yet there is little anger amongst its inhabitants, save that directed at the warriors of the Word Bearers who have driven them here.

This is to be expected.

The gates of Praxor have been closed for nearly two weeks, and tens of thousands of refugees fleeing the war and the doomed sun's radioactive spasms have sought sanctuary within. The shelter's accommodation is beyond its capacity, and the security of a weapons storage facility requires that every individual be identified. Once a full inventory has been taken of human and weaponised resources, a detailed campaign of resistance and reconquest can be developed.

Every entrance to the shelter, and there are many, has been sealed some with permacrete shuttering and some with warriors bearing guns. Elements from five different companies of the Ultramarines are now based here: five hundred and sixty-seven legionaries. They do not guard the entrances to the arcology. They train, they re-arm, they mount sorties onto the surface when word comes from Arcology X that enemy forces are nearby.

The security of the gates falls to the Imperial Army of which there are sixteen separate regiments present locally and skitarii elements swept into the arcology by the star's radiation. Command protocols and communications are still in disarray as the Mechanicum adepts try to mesh Army vox-systems with their own and those of the Legiones Astartes. Different systems, hundreds of encrypted networks and trillions of code combinations have brought a special kind of hell to operational co-ordination.

It is this that is giving Major Kadene a headache that is only getting worse.

She and her squad of Cardace Storm Troopers occupy one of the smaller routes to the surface, more accurately described as a sinkhole filled with hardscrabble that has been pulled apart by millennia of tectonic movement. It is, nevertheless, a passageway that connects the caverns below with the surface and must be guarded.

Temporary shuttering sprayed with rad-proof sealant allows unprotected humans to occupy the prefab guard post and barricades that watch for infiltrators from the surface. Twenty soldiers occupy the position: battered, war-brutalised veterans who have seen their world torn apart and broken into pieces that can never be put back together. Major Kadene's men have fought the good fight, and only these twenty of her seven hundred remain. They fought at the Pasuchne Bridge, and held it long enough for the 86th Company of the Ultramarines to cross. Along the Marusine Highway, a ten-thousand-strong rabble of cultist scum chased them for a hundred kilometres before they reached the regimental strongpoint set up at the Talanko Arterial.

Hol Beloth's flanking forces, moving to encircle Lanshear, were on the verge of forcing them to abandon the strongpoint. But then came the fiery rain from orbit, burning the Word Bearers and their rabble to vapour ghosts.

Leaving her company colours flying proud at Talanko, Major Kadene followed Colonel Rurik as he brought the scraps of their regiment to Praxor.

Kadene knows she will never see the surface of Calth, but hopes that some remnants of the enemy forces will try to fight their way into the shelter. She dislikes being underground, having discovered a mild claustrophobia, but she is a Storm Trooper, and to acknowledge weakness is not in her nature. She sits in the guard post's single structure, a reinforced tin shack, with a vox-caster and her unit's stock of anti-radiation pills, ammo, food and water. This is what has become of her once elite unit She flinches as a squawk of interference barks from the speaker horn of the vox-caster.

'Bloody Mechanicum,' says her adjutant, Corporal Bartebes. He smacks the grey-steel box with the heel of his palm. 'Bloody bastards never get anything bloody right.'

'I thought they were supposed to have this fixed by now.'

'And you bloody believed that, major?' says Bartebes, fishing a lho-stick from his pocket and lighting it with the ease of a professional. Oily smoke lifts from his mouth.

'I thought you quit,' says Kadene.

'I survived the surface,' replies Bartebes. 'If that ain't killed me, these bloody won't. It's boredom that'll do for me first.'

Kadene can't argue with his logic, and though she could order him to put it out, she won't. They have suffered too much in the last few weeks to deprive Bartebes of his vice. Besides, he's probably right.

She shrugs, turning on her heel as she hears the rumble of an engine. A big engine, something industrial. She wonders if there's something wrong with the sealant or the shuttering that requires a Pioneer work team. She doesn't feel any effects of surface radiation, but supposes that's probably why it's so dangerous.

'Now what's this bloody noise?' wonders Bartebes as a heavy industrial carrier lumbers around the corner. Its cargo compartment is draped with a blue tarpaulin, roped down and covering several objects, bulky and oblong in shape. Work tools? Engineering equipment?

'We expecting anyone?' asks Kadene.

'Not that they bloody told us,' replies Bartebes, giving the vox another clout. 'Not that we'd have heard on this piece of junk.'

The driver's Army, but she can't see his unit insignia. Thirty men accompany the carrier, some riding shotgun on the running boards, some marching alongside. They look bored, and Kadene can sympathise. There's something... ragged about these soldiers, but that's nothing unusual. Everyone looks a little ragged these days.

But her soldier's instincts are telling her there's more to it than that.

'Find out what they want,' says Kadene, lifting the vox-horn. 'I'll see if I can get some word from on high.'

Bartebes nods and reluctantly stubs out his lho-stick.

As he shoulders his hellgun, Kadene says, 'Eyes on.'

Bartebes understands immediately and his demeanour instantly changes.

He leaves the guard post and waves four soldiers to accompany him, bulky in glossy plates of ablative carapace. Each Storm Trooper wears the regimental insignia of crossed lances over a skull on one shoulder plate, a hand-painted black X on the other. With Bartebes at their head, they march out in front of the new arrivals. Bartebes waves his arms in front of him like a crew chief on a landing platform.

'Right, who the bloody hell are you?' he demands with his customary wit and charm. 'This is a Cardace post.'

A man in a uniform that hangs strangely on him detaches from the soldiers escorting the vehicle. He carries an old-style data-slate and holds it out to Bartebes. He says something she can't hear. Kadene lifts the vox-horn and twists the dial to the assigned command frequency.

As she does so, her eyes alight on a man partially obscured by the tarpaulin-wrapped cupola of the cargo vehicle. He wears armour, but it takes her a fraction of a second to realise what's wrong with it.

The man is dressed as a Cardace trooper, but she has never seen him before.

Her mouth opens to shout a warning.

A scream of dissonant noise erupts from the vox-horn, a blast of a million terrorised screams that comes from a place of horror and blood. It paralyses her. Literally paralyses her. Her every nerve is shrieking in pain, but she can't move.

Something pours from the vox-horn, a rush of stinking black fluid. It spatters the wall like an oil-filled balloon has just been thrown at it. She sees the men talking to Bartebes pull out flasks of black liquid and throw them to the ground.

She can't move. Fluid shapes leap from the black oil. She still can't move.

More glass breaks. More viscous darkness erupts like tarry geysers.

Shifting, formless things of grasping arms, gaping mouths and tearing claws slam into her soldiers and bear them to the ground. The rest of the men in her command drag their rifles to their shoulders, but there are shadows for them all. They slither over the floors, stretch and swell over the walls and loom down from the cavern roof. Men are plucked from the ground and black filth pours into their screaming mouths. It stops up their ears and noses, presses its way into their skulls through their eyes, and invades the entirety of their bodies in the space of a heartbeat.

Kadene sees all of this, but she still can't move. Her entire body is shocked rigid by the squalling blast of nerve-paralysing sonics. The vox is laughing at her. The spatter of oil on the wall is pushing itself into a semblance of form. Human, but larger than any man she has ever seen. Bulked out beyond mortal norms, she recognises the fluid-formed outline of Legion plate. The helmeted head has a horn that curls around it, and is formed from glistening matter that stinks like a mass grave.

It turns its gaze on her and she wishes she could close her eyes. She wants nothing more than to shut this abhorrent monster away.

The door to the guard post is thrown open. The man Bartebes was talking to enters.

'They're all dead,' he tells the horned black torso extruded from the wall.

Behind him, Kadene sees her men being stripped of their armour and uniforms. The killers garb themselves in the colours of a regiment that, but for her, is now extinct. The dishonour is beyond insult. It is violation.

'You know where to take the device?' asks the monster, its voice a gurgling wet horror of liquid vowels and drowning consonants.

The man nods. 'The statue of Konor in Leprium. Rendezvous at zero-dark-thirty.'

Kadene wants so badly to reach down for her holstered laspistol. Sweat beads on her forehead. Her hand trembles and, incredibly, she feels a tingling sensation in her fingertips.

'Take three men and dump the corpses at least five kilometres out,' says the black apparition. 'The defenders must not learn what was taken until it is too late.'

'It won't be long before a relief force turns up.'

The black shape gurgles with what Kadene realises with sick horror is laughter. 'You wear loyalist uniforms. Welcome them and share the camaraderie of brothers. Then kill them.'

The black shape on the wall turns to her. A slit of a mouth forms in its impossible helm, a leering grin of anticipation. She feels warm leather at her fingers. The holster is open; she never keeps the press-stud closed. Sweat pours down her face, veins stand out. Her hand shakes as she slides it around the weapon's grip.

'Such gross betrayal of trust has power beyond measure,' says the horned monster.

Kadene draws and fires her pistol with a scream of pain and grief. All she has already suffered and all she has just lost is distilled into this last act of defiance.

She shoots the monster again and again. Her bolts burn it like a solder through plastek and ignite it like promethium. It burns away into a stinking mist. A sulphurous reek fills the guard post, the stench of voided bowels. She tries to turn her pistol on the mortal traitor, but the weapon is slapped from her hand. A rifle butt slams into the side of her face. Bone breaks and she falls to the ground. Pain shoots around her body and a gut-cramping nausea stabs through her paralysis.

The traitor drops on top of her, one knee in the chest, another over her throat. He has a black-bladed knife in one hand, the tip scratching the surface of her eyeball. Fluid oozes out over her cornea. His palm rests on the dagger's pommel, ready to drive it home.

'Just for that, I think you're gonna come with us,' he says. 'Be interesting to see what your new sun does to one of its own.'

VIII.