As I close on the prone form, my suit lamps shine upon a scarred and shaven head. He brings up his chin and sputters the remaining water from his multi-lungs, and sharp, Colchisian features greet the illumination.

I halt in the shallows when I see his eyes. They are gone.

The flesh about the empty sockets is bloody and botched. His eyes have either been taken by another, or he has cut them out himself. The senseless barbarism and despoliation of the Emperor's flesh disgusts me.

The Word Bearer senses the movement about him and reaches out for my armoured leg.

'Friend?' he coughs.

I wade behind my enemy. My blade slips beneath the renegade's chin and rests against his inviting throat.

'Foe,' I correct him.

The Word Bearer finds his way to a smile.

I look to Tauro Nicodemus. 'At your command, my lord,' I say. The tetrarch does not look pleased.

'Sergeant,' he says. 'Where does that lake lead?'

'I was not under the impression that lakes led anywhere, tetrarch,' Arcadas replies.

'Tetrarch...' the Word Bearer mouths with obvious relish, until my sword presses harder into his Colchisian flesh.

'Those about to die have no business addressing princes,' I tell him. 'Now hold your tongue, or you'll force me to cut it out.'

'I fear you may merely end what he has started,' Nicodemus says, looking at the mutilation already wrought on the Word Bearer's face. 'What are these markings on his head?'

I look down at the hatch-scarring across the Word Bearer's shaven skull. It looks like a grate or portcullis.

'Exalted Gate Chapter,' I inform him. 'Just like Shax.'

The Word Bearer's pained smile broadens. I look to Nicodemus. 'It would be my honour to end this abomination now,' I say, echoing his earlier sentiment. 'However, I think it might be prudent to put questions to this prisoner.'

'Pelion...' the tetrarch warns. I am testing a hero's patience.

'The lake clearly leads somewhere my lord,' I say. 'The dark depths alone did not give birth to this aberrant brother.'

'I wouldn't bet on that,' Nicodemus mutters.

I turn to the tetrarch in a formal salute. 'Ungol Shax remains a threat, my lord. His men are operating in the region. He might be operating in the region. Surely, it would be tactically perilous to allow that? The prisoner might have information to that end. I request an interrogation-audience, Lord Nicodemus.'

Vexation ripples across his patrician features. 'Sergeant Arcadas,' he calls out.

'My lord.'

'Have your men complete their sweep of Tantoraem.'

'Yes, tetrarch.'

'In the meantime,' Nicodemus tells him, 'have a chamber cleared and set aside for the questioning of the prisoner.'

'Straight away, my lord.'

'Pelion,' the tetrarch says, turning to walk away. 'Have the prisoner gagged, secured and brought before me.'

'Sir?'

'I shall conduct the questioning myself,' Nicodemus says. 'Have no doubt, Honorarius Pelion, that if I suspect treachery of any breed or creed, I will order the prisoner ended information or not.'

I don't quite know what to say. I watch his scarlet cloak stream about him and follow the tetrarch into the darkness.

'Thank you, my lord,' I call after him.

The intruder is captured by Pelion and his brothers The chamber has clearly been used for sacrifices in the recent past. Splatters of browning blood forms a collage with other forms of filth across the walls, floor and ceiling. What Sergeant Arcadas had taken for some kind of stone table actually appears to be a rune-inscribed altar, loaded with profane ritual significance.

The Word Bearer doesn't know that he's seated before such an atrocity, blind as he is. I put him down harshly on an empty ammunition crate. He's unsteady, and not just because he can't see. I had summoned one of the engineer crews we used to secure and maintain barriers across the numerous arterial tunnels and arcology subways. Using their plasma torches, I had the Word Bearer's arms braced across his chestplate and the palms of his gauntlets fused to his armoured sides. So there the bastard sits: a prisoner in his own plate.

The bolt round rattles around the inside of my gauntlet.

Tauro Nicodemus stands before the prisoner, resplendent and grim in equal measure. Brother Daesenor stands sentinel on the doorway, the fat muzzle of his boltgun trained upon the prisoner. The tetrarch nods. I cut the gag from the Word Bearer's mouth with the tip of my sword.

The prisoner works his jaw.

'Name and rank,' Nicodemus demands. The Word Bearer purses his dark lips. 'Let's not play games, legionary,' the tetrarch insists. 'You know that I will not dishonour your flesh nor my own with torture and affliction. Let us talk as Legiones Astartes, as warriors of a galaxy broad and wide, and divided. As enemies, if you wish, but enemies that both hate and respect one other.'

'You have a gift with words, tetrarch,' the legionary observes with a smile. 'In another life, you might have been a bearer of the Word. Are you sure you have chosen the right side?'

'Of all the things we want from you,' I say from behind him, 'praise and approval are not among them.'

'Name and rank,' the tetrarch demands again.

'My name is Azul Gor,' the Word Bearer says. 'Exalted Gate Chapter. And you?'

'Tauro Nicodemus of Saramanth.'

'Oh, how the mighty have fallen,' Azul Gor says.

'The mighty go where they are needed,' Nicodemus counters. 'Today, I am needed on Calth. On another day it might be anywhere in Ultramar. On another still, anywhere in the Imperium of Man. Wherever my enemies dare to soil the earth with their presence, I will be needed.'

'I think it amusing that it was in fact the Warmaster that sent you to this doomed world.'

'Then Horus sent me to the place where I was most needed,' the tetrarch says. 'Perhaps there is hope for him yet.'

I interject. 'Galactic politics aside, I hope you don't mind me asking where you and your villainous kindred have been hiding. We paid you a visit. You were not at home.'

'I was in the deep and the dark,' Azul Gor replies absently.

'Can't we all say that?' I mutter.

'We cannot, Ultramarine,' he hisses. 'Imagine being blinded, stumbling about a cave as black as night, buried deep below the surface of dead world a world bathed in the glare of a star turned from the light. Can you imagine a deeper darkness?'

The chamber falls to silence.

'What happened to your eyes?' Nicodemus asks.

'I put them out,' Azul Gor said. His honesty burns. 'I put them out so that I might not have to look upon your starched faces and the dazzling gleam of your untested war-plate.'

'You didn't expect to find us in Tantoraem,' I accuse.

'And you negotiated a flooded cave system, without your weapon or helmet,' the tetrarch adds.

I nod. 'Or your eyes. I put it to you, Word Bearer you did not expect to find us at Tantoraem. I think you were looking for your master, Ungol Shax.'

The blind defector begins to laugh. It is a horrible chuckle laced with venom and bitterness.

'Ungol Shax is dead.'

'You lie!' I spit back, working my way around the altar. 'It is all you know. It is all you are. I would slit your throat, but for the untruth that would pour from the wound in place of good, honest Legion blood.'

'I wish you would, Ultramarine,' Azul Gor roars back.

I lash out. My blade lurches forward, coming to rest under the Word Bearer's sharp chin.

Nicodemus throws up his hands. 'Pelion!'

'Where is Ungol Shax?' I hiss.

'He is dead,' Azul Gor tells me once again, 'as I soon will be too. As will you be, Brother Pelion.'

'By your hand, I suppose,' I dare the Word Bearer.

'No,' he says. 'By my word. You roar your boldness, but sometimes actions speak louder. You restrain me here a blind prisoner with your blade at my throat and the clunk of a primed boltgun aimed at me from the corner. You stink of fear. Fear. That makes you weak. I need not blades nor boltguns. I have words, and I could end you with but a single one.'

'And which word would that be?' I furiously demand, the tip of my sword dimpling the flesh of his throat.

'Penetral'

The small chamber echoes with gunfire.

It is over. Azul Gor is dead. Three bolt rounds. Two in the chest, and one in the skull. Brother Daesenor's weapon smokes in the silence that follows.

I round on the sentinel, but Nicodemus raises a gauntlet.

'I ordered it,' the tetrarch admits, 'as I told you I would. This is my fault. This was a mistake.'

'He was talking,' I protest.

'He was,' Nicodemus agrees. 'He was talking you into the darkness. You've seen how far the Word Bearers have fallen. You've seen their depravities. That word was likely some kind of incantation, and his death at your hands would have been a latent bargain with some otherworldly creature.'

I stare at the tetrarch.

'We would do well not to underestimate our lost kinsmen,' he continues. 'The entire episode being unarmed, the eyes, emerging from hiding it was probably a ruse to get him into a room with an Ultramarines officer. A target worthy of his sacrifice. It is my fault. I take responsibility.'

The tetrarch goes to leave the chamber. He looks to Daesenor and nods at the trussed-up corpse of the Word Bearer. 'Take care of that please, brother,' he says, before turning to me. 'I'm going back to the Arcropolis. Have Sergeant Arcadas complete his sweep and then withdraw from this damned place. Assist the Army sappers in demolishing our breach point.'

'Won't you reconsider occupying the arcology?' I say, but my heart isn't in it.

The tetrarch ignores my words.

'Ensure that nothing can get through where we entered,' Nicodemus says. 'That's your responsibility.'

The breach point is nothing more than a ragged hole in the cavern wall.

Seismic demolition charges had been requisitioned from a tunnel-team lockup. They are not military grade, or anything close to the power and precision of the tactical demolitions used by the Legiones Astartes. However, in sufficient quantity and under expert supervision the seismic charges would do the job.

Sergeant Arcadas is clearing the last of his warriors from Arcology Tantoraem. With members of the Army, the sergeant's Space Marines had made swift work of searching the cave system for Legion munitions and power packs. All else rations, weaponry and plate was destroyed on the further orders of the tetrarch for fear it might somehow be contaminated. Blades were broken. Fibre bundles were ripped out. Bolters were breech-blown or fouled with crude plugs.

Imperial Army forces trudge by under the milky orb of Sergeant Brotus Grodin, carrying caches of recovered munitions and packs. Grodin is a retired soldier one of the Emperor's ex-serviceman, who has been placed in charge of one of the newly organised units of the Veridian Cicatrix. The Cicatrix had been the tetrarch's idea: Cicatricians are all remnants of former defence regiments that have been decimated and scattered during the surface war. Their camo-chitons are a myriad of local colour, each member hailing from a different defence force or ceremonial guard. All wear flak plate from Konor breastplates, skirts and guards. Their visored helms display the nose and cheekguards favoured by many of the Calth militia, and each carries a battered buckler, short blade and the slung length of a las-fusil.

Their exposed forearms and thighs all bear horrific radiation burns and solar scarring. This is the now infamous Mark of Calth, a testament to their desire to fight on across the sun-scorched surface of their doomed home world. It was this unifying feature that Nicodemus chose to honour in their name, despite the fact that Grodin's contingent alone is made up of former members of the Vospherus 14th and 55th Irregulars, and the Tarxis 1st Citizen's Reserve. Helmetless, with the scowl on his roasted half-face driving the Cicatricians on, Grodin taps the passing soldiers on the arm with a swagger-sceptre.

'All through, m'lord,' Grodin reports gruffly.

'Thank you, sergeant,' I say. 'Would you be so good as to accompany my legionary brothers back to the Arcropolis with the supplies?'

Grodin nods and follows his dour troops, leaving me with Brothers Daesenor and Phornax as breach sentries.

Ione Dodona also remains.

She retreats, unspooling detonator cable. The three of us follow her to an outcrop, behind which she has set up a simple plunge-detonator. The equipment is only frontier mining-standard, but serviceable like the seismic charges Dodona is using to collapse the breach point.

'Are we set?' I ask.

'Two more charges to wire,' she answers, fingering through the nest of cable. 'One more minute.'

Dodona has been invaluable. Grodin's men have heart and grim determination but they are all topsiders. As a Sapper Second-Class, even before the conflict, Dodona had been part of the Calth Pioneer Auxilia. Commonly known as 'the Benthals', the sappers' expert knowledge of the cave systems, structural integrities and explosives became a powerful weapon in the war as it progressed beyond a simply military endeavour. Many lives and much in the way of precious ammunition have been saved by the strategic collapsing of caves and tunnels swarming with cultist forces and degenerate Word Bearers.

Collectively, Dodona possibly has a higher kill-count than some frontline battle-brothers. What they achieve with bolt and blade, the sappers accomplished with millions of tonnes of rock. In a way, Calth itself has taken the fight to the invaders.

As we wait, Daesenor and Phornax monitor the breach for enemy activity. Without the opportunity to carry out a full survey, there was no way of knowing all of the entry and exit points in the Tantoraem arcology. An enemy force could stream through and flood our territory through our own breach point. My brothers' bolters are there to give Dodona time to finish her work and bury any opportunists. As it is, all is silent and still.

Casting my eyes across Dodona's equipment and schemata, I pick up a scratched dataslate. It displays detailed maps of arcologies both completed and before the war in a state of construction. Tracing my ceramite fingertip across the slate, I follow the pillar-lined mag-lev tunnels out of Magnesi-South, through the breach point and down through the branching cave systems of Tantoraem. My digit drifts the torturous route of our incursion. I think on the brothers lost under my command, drowning in the sea of rabid cultists. I feel my boots slipping in the blood of our loyal Cicatricians, and relive the clash of our formations against throngs of fanatical Word Bearers, like ships smashing against rocks in the shallows.

Then I reach the groundwater lake, the shallows where we captured Azul Gor. To my surprise, my finger travels on, arriving at a single slate designation: Penetralia.

'What is this?' I ask Dodona, who is clearly not impressed at having to disentangle herself from detonator cables to check the slate. Unlike the Cicatricians, her lamped helmet is close-fitting and her flak-plates are set into a dark body-suit, better adapted to clambering through rough caves and tight tunnels. She shines her lamps down onto the slate screen.

'That would be the Penetralia,' she tells me. 'It's a series of tunnels formed naturally in the rock. It's quite a labyrinth down there, but the region was ear-marked for excavation as the entry point to another arcology.'