'Having second thoughts?' asked Vaanes.

'No,' said Uriel.

'Sure?'

'I'm sure, Vaanes. We can do this. It will not be easy, but we can do it.'

Vaanes looked unconvinced, but pointed to where the plateau narrowed to become a near-vertical shear in the rock that carved a path down the flank of the mountain, 'That's the way down that leads to the plains below. It's steep, very steep, and if you fall you're dead.'

'How the hell are we meant to get down that?' breathed Leonid.

'Very carefully,' said Vaanes. 'So don't fall.'

'It's all right for you,' said Ellard, slinging his lasgun and making his way towards the path. 'If you fall you have a jump pack!'

'What? You want me to announce our presence here?' returned Vaanes.

Uriel followed the renegade and was seized by a dizzying lurch of vertigo as he saw the route they must take.

The plain was thousands of metres below them, steaming waterfalls of molten metal splashing along basalt channels towards lakes of glowing orange below.

'You need to go down facing the rock,' explained Vaanes, edging onto the path, barely half a metre across and gripping onto cracks in the rock for handholds. Gingerly, he edged out onto the path, leaning into the rockface and sliding sideways along and down.

Uriel went next, gripping the rockface and easing himself out onto the narrow path. He kept his weight forward, knowing that to overbalance even a little would send him plummeting thousands of metres to his death. Cold wind whipped at him and he felt his heartbeats hammering in his chest.

He edged out, following Vaanes's example and utilising the same handholds wherever he could. Within the space of a few hours, his muscles ached, his fingers burned with fatigue and they were barely halfway down. His breath came in short, hard gasps and it was all he could do to not look down.

Hand over hand followed hand over hand and shuffling step to the side followed shuffling step to the side until they reached a point where the slope became shallower and it was possible to climb directly downwards for a short distance.

As Uriel climbed down to a narrow ledge, he flexed his fingers, the textured pads of his gauntlets torn and useless. His arms were leaden weights and he hoped he had the strength to make it to the bottom. With a little more room to manoeuvre on the ledge he carefully eased round and gazed at the terrifying scale of the siegeworks below.

What had brought this siege about anyway? Was it some internecine conflict or was there some other, darker purpose to the slaughter going on below?

Did the attackers have some knowledge of the Heart of Blood or the daemonculaba?

He supposed it didn't matter why the followers of the Dark Powers made war upon one another: the more they killed each other, the fewer were left to attack the Emperor's realm.

A startled cry from above snapped him from his reverie and he looked up in time to see a hail of stones skitter down the slope, closely followed by Colonel Leonid, who screamed in terror as he tumbled downwards.

Uriel pressed himself flat against the rockface and leaned dangerously to one side to snatch at Leonid as he plummeted past.

His fingers closed on Leonid's uniform jacket and he gritted his teeth, gripping the rocks tightly as the colonel's weight threatened to pull them both from the ledge. Under normal circumstances, Uriel would have had no problem with catching Leonid like this, but off balance on the edge of a crumbling corbel of rock he felt himself being pulled from the cliff as his agonised fingers slipped from their transient handhold.

'I can't hold on!' he yelled. The ledge crumbled at the edge, dirt and pebbles spiralling downwards to the plains far below.

'Don't let go!' screamed Leonid. 'Please!'

Uriel fought to hold on, but knew that he could not. Should he just let go? Surely the presence of Leonid would not affect their mission one way or another. He was a normal man amongst Space Marines, what good could he possibly do?

But before he could release his grip he felt a hand take hold of his shoulder guard and pull him back. Above him, Sergeant Ellard had hold of his armour and strained to pull him back. Uriel was too heavy for him to hold, but Ellard's strength was prodigious and held Uriel long enough for him to shift his grip to a better handhold with firmer balance. Centimetre by centimetre, Uriel eased himself back onto the firmer ground of the ledge and was able to deposit Leonid back onto the slope.

The colonel was hyperventilating, his face pallid from shock and terror.

'You are safe now, Mikhail,' said Uriel, deliberately using the colonel's first name.

Leonid took great gulps of air, keeping his eyes averted from the drop behind him. His body shook, but he said, 'Thank you.'

Uriel did not reply, but looked up to see a breathless Sergeant Ellard clinging to the rockface by what looked like his fingernails. Uriel respectfully nodded at the man, who nodded back.

'Sir, are you able to go on?' asked Ellard.

'Aye…' wheezed Leonid. 'I'll be all right, just give me a minute or two.'

The three of them waited as long as they dared before moving onwards, Uriel in the lead with Ellard bringing up the rear. The colonel's steps were hesitant and unsure at first, but eventually his confidence returned and he made good time.

The journey down the mountains blurred into a painful series of vignettes: traverses across terrifyingly narrow spurs of rock and heart-pounding drops onto splintered ledges. Uriel continued down the slope of the mountain, pressing himself flat against the rock until he felt a tap on his shoulder and looked around to see that he had reached the base of the shear in the rock, that he was on a wide, screed slope of ash and iron debris. A churned mass of broken earth sloped gently to the darkened plains below.

The warrior band were spread around, breathless from their climb, and as Uriel looked up to see Leonid and Ellard completing the descent, his admiration for their endurance and courage soared as did his shame at the thought of even considering letting Leonid fall to his death.

Ardaric Vaanes approached him and said, 'You made it then.'

'You were right,' said Uriel. 'That was not easy.'

'No, but we're all here. Now what?'

That was a very good question. They were still many kilometres from the fortress, and Uriel could not even begin to guess how many enemy soldiers lay between them and its lower slopes. He scanned the ground below him, picking out scores of work parties and earth-moving machines hauling hundreds of tonnes of earth to build the ramp that led towards the fortress. A hissing lake of molten metal pooled at the base of the slope, bathing everything in a hellish orange glow and the rumble of engines and cursing voices drifted up from the construction sites.

'You know there's no way we can just walk through that many soldiers. Even if the vast majority are only human.'

'I know,' replied Uriel, eyeing the huge bulk-haulers. 'But perhaps we will not need to.'

The heat radiating from the molten lake was stifling, filling the air with stinking fumes and making each breath hot and painful. Uriel edged around a tall mound of piled steel sheeting and waited for the latest work party to shuffle past, chained together at the neck by spiked collars and dressed in filthy rags. Servants of the Iron Warriors in all-enclosing vacuum suits shouted gurgled commands to the slaves, beating and whipping them as they pleased.

The rumble of heavy, tracked bulk-haulers and booming gunfire covered the Space Marines' approach down the lower slopes of the mountain, the darkness of the smoky clouds only helping them to approach the construction site unobserved. The huge machines were bigger than the largest super-heavy tank Uriel had ever seen, controlled via a cab mounted high on a massive, tracked engine unit that pulled a huge container on wheels with the diameter of three tall men.