The heavy adamantium gates of Arcology X rumble closed on rollers the size of Land Raiders, blotting out the venomous blue light of the system star. Booming locks hammer home, shutting the subterranean complex off from the upper world. Thundering recyc-units purge the contaminated dust from the vast airlock chamber.

Remus Ventanus and the warriors of the Fourth Company stand immobile in the roaring winds as a Mechanicum adept and a host of servitors with hostile environment augmetics come forward with high pressure hoses, to scour them with electrolysed water that runs into specially dug sluices.

Ventanus has little patience for such processes, but with so many mortals packed into Arcology X, decontamination is a necessary evil of any mission to the surface.

Selaton and Barkha stand behind him, Barka still clutching the battered pole of the company standard that Ventanus retrieved from the slaughtered honour guard at the Numinus starport. Water drips from the eagle and the Ultima, making both shine brightly in the gloom of the gateway. The symbolism pleases Ventanus and makes the time taken to cleanse their armour feel worthwhile.

He could have the indentations of the dead warrior's grip worked out of the metal, but he will not. The dying grip of the Ultra-marine whose name he never knew will be a constant reminder of the Word Bearers' betrayal.

Wherever this standard ends its days, it will forever display the mark of its former bearer.

With the closing of the gate and the completion of decontamination procedures, the defence protocols ease a fraction and servitor-crewed turrets switch their macro-weapons from armed to safe. An internal bulkhead the size of a jungle escarpment rumbles down into the floor with the sound of tectonic plates grinding together. The Mechanicum adept waves the Ultramarines in and leads his servitors away.

Ventanus marches from the decontamination barbican into Arcology X.

IX.

Captain Octavian Bruscius makes his way through the neatly arranged lines of beds and temporary shelters housing the groups of civilian survivors packed into CV427/Praxor. Bruscius is a gene-forged post-human, and they are but mortals, yet they are all warriors of Ultramar.

He feels proud to number himself among them.

He has fought in the Legion's battle lines for a century and a half, but fighting within the bounds of the Five Hundred Worlds is something he never expected.

There has never officially been a theoretical for a war between the Legions, and though Bruscius understands he is simply a line officer, even he recognises that the Ultramarines will never be the same again.

The Warmaster's treachery has upset the order of the galaxy, and nothing will ever be the same. He and his battle-brothers of the 24th Company are based in Praxor. Warriors from the 56th, 33rd, 111th and 29th Companies are here too. His group is the largest, boasting two hundred and nine warriors, whereas the 111th has been reduced to a single squad.

Cut off from the fighting, they are isolated beneath the tumbled ruins that are all that remains of the Persphys and Caela Praefecture conurbs.

Techmarine Colbya has established contact with sixteen other nearby shelters, as well as Captain Ventanus in what is now known as Arcology X. Bruscius does not know why its name has been changed, and is just glad that Lord Guilliman saw fit to place Ventanus in control of the fight-back.

The Word Bearers have taken the surface, but the war beneath belongs to the Ultramarines.

Bruscius forces himself to keep such thoughts tamped down.

He has more immediate concerns.

Registration areas have been set up in each of the largest caverns, manned by the few Administratum personnel who escaped below ground. It is a thankless duty, but the citizens of Calth form snaking lines as they await their turn to be processed without complaint. Well over ten thousand people are crammed into this cavern alone, with more pressing in behind. Motorised gurneys drive along cleared lanes between the queues, bearing reams of accumulating paperwork and identity confirmations from the registration booths. The drivers are all Army-helmeted and with rifles slung across their backs.

Bruscius and twenty of his warriors are here to oversee the registration process and watch for any security breaches, but his company will be rotated onto surface patrol soon. He recognises the importance of this work, but Bruscius wants to kill Word Bearers.

His eyes roam over the thousands of people in the cavern, pleased to note the stoic determination on every face. These people have seen their world virtually destroyed, but there is no trace of panic or psychosis. They came with nothing but that which they could carry on their backs when the evacuation order came through, yet still stand proud and ready to serve.

What other citizenry of the Imperium could rally so magnificently?

Almost all are young. All are ragged and grimy. But no amount of dirt can hide the mottled purple radiation burns with which almost every man, woman and child's skin is afflicted. The medicae call it the 'Mark of Calth', and it is as much a badge of honour as it is an injury.

Bruscius moves on, traversing the echoing cavern and counting the hours until he can turn his weapons on the enemy. Everywhere he goes, people turn to stare at him, and he finds the attention faintly discomfiting. He is a warrior, pure and simple, yet these people invest him with all their hopes of a better tomorrow.

It is a heavy burden to bear, one he had not known he shouldered until this moment.

A woman with a babe in arms clutched tight to her breast approaches him and she reaches out to touch his vambrace. Under normal circumstances, Bruscius would never allow such contact, but these are far from normal circumstances. Another two children hold tight to the hem of her skirt, both so young and fragile looking that Bruscius finds it hard to believe they survived the horrors above.

'Emperor protect you,' she says.

Bruscius does not know how to respond and gives the woman a nod. She smiles, and he knows she will treasure the memory for the rest of her days.

The Ultramarines have become touchstones of hope, living proof that Calth will rise again, that its people will one day reclaim what was taken from them. It has been a humbling experience, and a salient reminder of why the Great Crusade was fought in the first place.

The woman holds out her hand, and Bruscius sees a small aquila pendant on a silver chain lying flat against her palm.

'Take it,' she says. 'Please. You have to.'

The Ultramarines have standing orders not to accept gifts from civilians. Despite that, their muster spaces and arming points are surrounded by offerings, tokens of gratitude and handwritten messages declaring a readiness to fight for Calth.

'My thanks, but it is not permitted,' he says, turning away to move on.

'Please,' says the woman, more insistently. 'She needs you to have it.'

Something in the woman's tone makes him stop and turn back to her.

'Who needs me to have it?' he asks.

The woman tilts her head to the side, as though confused at his question.

'The saint,' she says, almost in tears. 'You need to see. Before it is too late.'

Bruscius finds himself reaching for the aquila, though he knows he should not. The woman sighs as though a pent-up breath has just been expelled from her lungs. She looks up at him, and though Bruscius does not easily recognise conventional human expressions, he sees she is surprised to find herself face to face with a Space Marine.

As his hand closes on the silver pendant, combat reactions surge within his post-human body as chem-shunts within his battle armour flood his system with combat stimms in expectation of battle. His bolter snaps up and his visor is suddenly overlaid with tactical schemata, spatial signifiers and topographical data.

A vox-link instantly activates between him and his battle-brothers.

Bruscius has no idea what has triggered this reaction and the woman backs away from him in fear as he goes from heroic saviour to lethal, bio-engineered killer in the blink of an eye. He scans for any sign of threat and immediately sees the motorised gurney bearing boxes of administrative documents and the like.

Two things are immediately obvious.

First, the gurney is laden with heavy boxes, but is heading towards the registration booths.

Second, its driver wears Army fatigues, but they are ill-fitting and clearly not his own.

Bruscius sets off at a run towards the gurney, bellowing for people to get out of his way as a terrible foreboding fills him. The driver sees him coming and grins with zealous fury as he halts the gurney in the centre of the cavern.

Bruscius pulls his boltgun tight to his shoulder. A targeting reticule fastens on the man's centre mass. It flashes red in full expectation of a lethal shot. The man stands and shouts at the top of his voice, with his rifle and a black-bladed dagger held aloft.

'Hear the Word of Lorgar!'

It is all he manages before Bruscius's mass-reactive blows out his chest and entire upper body in a wet meat explosion. People duck for cover, clearing a path for Bruscius as his warriors close on his position.

'Get back!' shouts Bruscius, kicking the dead man's remains from the driver's seat and hauling boxes from the back of the gurney. As he feared, they were concealment for something hidden behind them a long, crudely-machined tube of thick metal, sealed at both ends by seamed welds and pierced by a multitude of sheathed connection jacks, electrical buffers and decoy wires. Behind a crystalflex panel, Bruscius sees a pair of brushed steel casings marked with the symbols of his Legion.

His armour registers a blazing spike of radiation, but it is the only warning Bruscius gets.

The stolen atomics detonate a second later, filling the cavern with nuclear fire that spreads through the entirety of Shelter CV427/Praxor and kills every living soul within.

It is the first of three such atrocities that murder two million civilians in one night.

X.

It still amuses Ventanus that a mark he made in haste upon a wax-paper map has become so synonymous with the defenders of Calth. With Lanshear laid waste by the orbital batteries, the defenders had needed a place to rally. With virtually every data-engine on the planet dead, a pict scan was made of Ventanus's map with a rally point marked with black ash.

That scan was broadcast through every civilian pict-caster and Legion slate within reach of Lanshear, and thus was named this bastion of resistance.

Arcology X.

Two quick, crosswise slashes on a map and an element of geography became a piece of history.

A symbol of resistance and a talisman to brandish in the face of the enemy.

XI.

The caverns are dim. Power consumption is carefully controlled. The few Mechanicum adepts have yet to stabilise a link to the geothermal grid at the heart of Calth. Flickering lumen globes in protective caging are strung from brickwork supports on looping cables like jungle creepers. This close to the surface, the architecture has a martial character, but with every sub-level they traverse, the more civic and functional it becomes.

The walls are etched with metres-high Xs, and hundreds more on every archway and lintel. Among them, Ventanus sees pictures drawn on the walls, serpentine creatures with dark wings and fanged mouths. Draconis. He sees a childishness to the scratched lines and wonders if these nightmares have been drawn on the walls as a means of expelling them. Are they memories of the monsters brought forth by the heinous pacts made by the Word Bearers or visions drawn from the nightmares common in the wake of the attack?

News of Ventanus's mission has already reached Arcology X, and the return of the Fourth is greeted with cheers and loud huzzahs from the thousands of civilians packed into its sprawling sub-levels. Someone shouts the word saviour, and the cry is taken up by the multitudes packed into the caves. It follows them down the levels as they plunge deeper and deeper into the bedrock of Calth.

Sydance is waiting for them at the gateway to the administration levels.

His cobalt-blue armour is clean and polished. Some of the Legion have made oaths not to remove the dust and blood of war until Calth is reclaimed, but like Ventanus, Lyros Sydance wants the Word Bearers to see the Ultramarines are still the regal Battle Kings of Macragge.

No amount of treachery and no grief will ever change that.

But even Sydance has adopted the black X on his shoulder guard, carefully etched between the curved arms of the marbled Ultima. It looks like a Chapter number or a company designation, but it is something far more important.

'You're making a name for yourself down here, Remus,' says Sydance as the chants continue behind them.

'Nothing to do with me, Lyros,' replies Ventanus. 'This has your fingerprints all over it.'

Sydance shrugs and grips Ventanus's wrist. 'A bit of hope and glory never hurt anyone.'

Ventanus does not release Sydance's arm. 'I want it to stop.'

'Why? What you're doing, it's giving people hope.'

'I'm not a saviour,' says Ventanus. 'And I don't like the connotations of the word.'

'You don't have to like it, you just have to endure it.' says Sydance, turning and making his way down the ramp into the cavern. 'Come on, the Server's waiting for you at the Ultimus.'

The war for Calth is being co-ordinated from the lowest level of Arcology X, a cavern seared from the lithosphere by melta drills and seismic charges. Beneath the levels of habitation, engineering and hydroponics, it is a rock-clad dome, some three kilometres in diameter, with numerous branching passageways, sub-galleries, and twisting dead ends radiating from its central void. At its heart stands a structure of polished marble and glass, utilitarian in elevation, but designed in the shape of the XIII Legion's sigil. Armoured panels encase its lower levels, and Techmarines aboard Tekton-pattern Rhinos work side by side with Mechanicum servitors to transform it into something resembling a strongpoint.

Before the invasion, the building was owned by a trading cartel founded in the time of Guilliman's adoptive father. It is named Konor's Arch, but is now known as the Ultimus. Its robust infrastructure and powerful data-engines designed to link subsidiary operations across the Five Hundred Worlds make it the perfect base from which to conduct offensive operations against the remaining Word Bearers.

Such concerns are vital, but once again the symbolism of the structure is paramount.

Hundreds of temporary structures surround the Ultimus, overspill from the levels above. So great were the numbers of refugees fleeing Lanshear that the upper levels quickly filled, and Ventanus had no option but to allow billets to be set up around his command post. He doesn't like it, but has little choice in the matter. There is simply nowhere else for them to go.

Word of their coming has reached the refugees, and people cluster at the edge of the clearway that leads to the gates of the Ultimus. People cheer and wave and clap. They shout his name, and once again call him saviour. He keeps his expression neutral, but catches sight of Sydance's amusement.

'You might not like the connotation, but The Saviour of Calth has a nice ring to it,' says Sydance. 'It's a title that'll stick, mark my words.'

'So what do they call you?'

'I haven't decided yet,' says Sydance with a grin. 'But we'll all have titles by the end of this.'

Ventanus walks on. He knows Sydance is right, but it still irks him to have the mantle of saviour thrust upon him. He dislikes the self-aggrandisement and its faintly theological undertones, but is canny enough to know that nothing he can do now will stop its spread.

'So, are you going to say it?' asks Sydance.

'Say what?'

'That you were right after all, and that I was wrong.'

'I don't need to,' says Ventanus. 'The truth is self-evident. Six hundred Word Bearers dead without the loss of a single warrior.'

'Yes, very impressive,' agrees Sydance, placing two fingers to his forehead and narrowing his eyes as though in a trance. 'I see many laurels in your future, great statues built in your likeness and a name that echoes through eternity.'

Ventanus allows a thin smile to surface. 'I will shoot you if you use those psychic powers again.'

Sydance laughs and turns from Ventanus and addresses the two sergeants behind him.

'Barkha, Selaton, good job.'

The sergeants acknowledge his words, but do not reply.

Ventanus looks up and sees Server Tawren and her newly-acquired retinue of lexmechanics, calculus-logi and data-savants approaching. He is still learning the nuances of human interactions something forced upon him by increased contact with the populace of Calth in recent weeks but has become familiar with the hybrid machine/flesh expressions of the Mechanicum.

Tawren has the chimeric qualities common to the members of the Martian priesthood detachment, aloofness and a disconnect that some see as cold but right now Ventanus sees nothing of detachment, nothing of disconnect.

What he sees in Tawren's face is an abyss of all too human despair.

'Something has happened,' he says. 'What is it?'

'CV427/Praxor is gone,' says Tawren. 'Two others as well.'

'Gone?' he says. 'What does that mean?'

'It means that they are radioactive craters hundreds of kilometres wide,' says Tawren.