Uriel crossed the chamber to join Pasanius, stepping over the bones of the dead as he noticed something metallic lying partially buried in the dust. He bent down to retrieve it, his fingers closing on a crude, thick-handled knife, the blade long and flexible. He turned to look at the splayed bodies and a sickening realisation came to him.

'They were skinned,' he said.

'What?'

'The bodies,' said Uriel, holding up the knife. 'They were skinned. They were killed and then their killers flayed them.'

Pasanius cursed. 'Is there no end to this world's evil?'

Uriel snapped the blade of the skinning knife and hurled it away from him, the broken pieces clattering from the rocky walls of the cave. What manner of beast would track its prey deep into the mountains to attack with such speed and frenzy before taking the time to remove its victims' hides? He hoped they would not find out, but a sinking feeling in his gut told him that there was a good chance they had already stumbled into its territory.

'There's nothing we can do for them, now, whoever they were,' he said.

'No,' agreed Pasanius. 'So which way onwards?'

Uriel crossed the cave, stopping to examine each passageway and hoping to discern some clue as to which direction offered the most hope of a way out.

'There are tracks leading away at this one,' he said, kneeling and examining the ground at the middle passage. 'A lot of them.'

Pasanius joined him, tracing the outline of a huge footprint in the dust. There was no telling how old it was, but, despite its size, there was no doubt that it was human.

'Are you thinking these might lead to the monsters' lair and that we should avoid it?'

'No, I think that they might lead to a way out of these tunnels,' said Uriel.

'I knew you were going to say that,' sighed Pasanius.

Uriel and Pasanius set off down the tunnel, its course meandering through the mountains for what felt like many kilometres, until they completely lost track of which way they were headed. As the ground underfoot became rockier, the tracks vanished and Uriel knew they were hopelessly lost.

But just as he began to think that they might never again see the surface - not an unappealing prospect in itself - he caught a hint of something on the air. A breath of motion, the faintest gust of a breeze on his skin.

He held up his hand and quieted Pasanius as he opened his mouth to speak.

Just on the threshold of audibility he could hear a soft rumble, like a distant crackle of white noise. Though it took all his concentration, he followed a twisting path through the tunnels, doubling back, stopping and retracing his steps every now and then as he followed the noise.

As it grew louder, his path became surer and within an hour of first hearing the noise, he saw a bright sliver of white sky ahead.

'I never thought I would be grateful to see that sky again,' said Uriel.

'Nor I, but it is better than that accursed darkness.'

Uriel nodded and the two Space Marines emerged from the tunnel, blinking in the perpetual daylight of Medrengard. As they stepped out onto the mountainside, Uriel saw the source of the noise he had been following.

'Guilliman's oath!' swore Pasanius.

Many kilometres ahead over the mountain was a fortification built from dark madness and standing in defiance of all reason. Its steepled towers and mighty bastions wounded the sky, its massive gateway a snarling void. Its walls were darkened, bloodstained stone, veined with unnatural colours that should not exist and which burned themselves upon the retina.

Lightning leapt between its towers and the clanking of great engines and machines echoed like thunder from beyond its walls.

Pillars of smoke and fire leapt from the walls where explosions blossomed against them, hurling great chunks of black stone from the colossal fortress. The distant rumble of artillery crashed and boomed, bright muzzle flares of innumerable great howitzers and siege guns firing upon the fastness from the jagged rocks below.

The primal battle cries of thousands, tens of thousands of warriors - perhaps even more - were carried on the wind from the distant battle, together with the smell of burnt iron and war.

Clouds of ash and smoke from the blazing pyres surrounding the fortress flickered and twitched with the fury of the siege below, and Uriel felt his soul blacken in the face of such savagery.

Nothing could reach that fortress and live.

But that was exactly what they had to do.

PART TWO

BENEATH A BLACK SUN

CHAPTER FIVE

A blast of superheated air whooshed between the stumps of the merlons, hurling Honsou from his feet and vaporising the top half of one of his Iron Warriors. He rolled to one side as the smoking legs collapsed beside him and leapt to his feet, leaning over the ragged remains of the fortress wall and waving his mighty toothed axe.

'Come on, Berossus, you will need to do better than that!' he shouted.

Far below, the metallic coughs of massed artillery fire echoed from the dark mountains, shelling the lower bastions of Khalan-Ghol to oblivion. The screams of dying men drifted up towards him, but Honsou paid them no mind. They were but slaves and those too badly injured for skinning in the flesh camps, and there were plenty more of them to expend.

He wiped dust from his armour as more Iron Warriors marched forward to plug the gap the stray shot had blasted in the upper levels of his fortress. It had been a lucky impact and Honsou felt a thrill of adrenaline course through his body at the near miss. Ever since the siege on Hydra Cordatus, he had craved the fire and thunder of battle once more. The fighting on Perdictor II upon his return to the Eye of Terror had been desultory and unsatisfying, the warriors of the Despoiler proving no match for his advance forces.

But now his ''fellow'' warsmiths were attacking him, and this was sure to be a battle worthy of the name. Once again he was forced to prove his mettle to those who thought him no better than the Imperial dogs they fought the Long War against. The bile rose in his throat at the thought that even though his predecessor had named him warsmith, he was still not considered their equal.

'Lord Berossus is thorough in his attentions,' said Obax Zakayo, his grating, static-laced voice snapping Honsou from his bitter reverie. 'The lower bastions will be nothing but dust and bones soon.'

Honsou turned to face his lieutenant, a huge, wide-shouldered Iron Warrior with yellow and black chevrons edging the plates of his dented power armour. Hissing pipes wheezed from every joint, leaking stinking black fluids and venting puffs of steam with his every step. Like Honsou, he carried a fearsome war-axe, but he also wielded a crackling energy whip, writhing on the end of a mechanised claw attached to his back.

'If Berossus thinks he is achieving anything by killing such chaff, then he is even stupider than I believed,' sneered Honsou, wiping grey dust from his visor with his glossy black augmetic arm. His former master had gifted the mechanical arm to him after his own had been hewn from his body by the late castellan of Hydra Cordatus. It had once belonged to Kortrish, a mighty champion of ancient days and had been a physical indication of his master's favour.

'What he lacks in imagination, he makes up for with determination,' said Honsou's personal champion, a tall, slender warrior in power armour so dark and non-reflective that he moved like a liquid shadow. His voice was a ghostly monotone, his face a crawling mass of bio-organic circuitry that ran like mercurial fire beneath his dead skin and made his eyes shine with a lifeless, silver sheen.