Attacking was a remarkably easy thing to do. Dorn's physical prowess humbled all but twenty human beings in creation, and those twenty were his father and his nineteen brothers. In Dorn's opinion, the real art was knowing when not to attack. His grandfather, the old Inwit sire, patriarch of the ice-hive clan, had taught him that.

Dorn had been the seventh lost son to be reclaimed. By the time his father's forces found him, he had become a system warlord in his own right, ruling the Inwit Cluster as the head of the House of Dorn. His grandfather had been dead forty winters, but still the warlord had slept with the fur-edged robe across his body at night. His people had called him ''emperor'' until the true meaning of that title had been demonstrated by a thousand warships in the Inwit sky. Dorn had gone out to meet his father aboard Phalanx, one ship against thousands, but what a ship: a fortress. His father had been impressed. Dorn had always excelled in the construction of fortresses.

That was why Dorn had returned to Terra with his gene-sire. Out of love, out of devotion, out of obedience, yes, but most of all, out of necessity, damn Singh. The stars had turned over, and Chaos had spilled out from under them. The brightest of all had fallen and the unthinkable, the heretical, had become fact.

The Imperium was attacking itself. The Warmaster, for reasons Dorn was quite at a loss to fathom, had turned upon their father, and was committing his forces to all-out war. That war would come to Terra. There was no question.

It would come. Terra needed to be ready. The palace needed to be ready. His father had asked him, as a personal boon, to return to Terra and fortify it for war.

No better man for the task. No better master of defences. Dorn and his Fists, appointed the Emperor's praetorians, could fend off any attack.

Below him, the halls of Terra were silent, and the walls deep. The only sound was the distant, eternal hum of the Astronomican. The Palace Dorn had armoured and defaced sat like a dark crown on the top of the world.

Rogal Dorn had built many of the finest strongholds in creation: the city fortresses on Zavamunda, the pylon spire of Gallant, the donjons along the Ruthan Marches. Impregnable bastions all, palaces for governor lords to rule from. None of them had been so essential as this fortification. None of them had been as painful to accomplish. It had been like blotting out the light or draining a sea. The bright glory of his father's triumph, the enduring monument to Unity, had been entombed inside a crude shell of utilitarian defence.

All because of Horus, because of the brightest bastard son, the bringer of new strife.

Dorn heard stone splinter. He looked down, he had punched his fist, his Imperial fist, through a block of stone in the parapet. He had barely registered the impact. The block was pulverised.

'My lord, is everything all right?'

Archamus had shadowed him from the planning chamber. Never so volatile as Sigismund, Archamus was the master of Dorn's huscarl retinue.

There was a worried look on Archamus's face.

'Just venting my emotions,' Dorn said.

Archamus regarded the splintered block. 'Making work for Singh's artisans, then?'

'Something like that.'

Archamus nodded. He hesitated, and looked out over the high walls towards the distant earthworks of the Mahabarat. 'You have wrought a wonder, you know.'

'I have ruined one.'

'I know you hate it, but it had to be so. And no one could have done it belter.'

Dorn sighed. 'You're kind, old friend, but my heart is lead. This should never have been necessary. I search the limits of my imagination, and still I can conceive of nothing that begins to explain this war. Pride and ambition, insult, jealousy? They are not enough, not nearly enough, not for a primarch. They are too petty and mortal to drive a primarch to such extremity. They might provoke an argument, a feud at the worst. They would not split the galaxy in half.' Dorn looked up at the night sky. 'And yet, against all reason, he comes.'

'Guilliman will stop him.'

'Roboute is far away.'

'Russ, then. The Lion. The Khan.'

Dorn shook his head. 'I don't think they'll stop him either. I think he'll roll on until he reaches us.'

'Then we'll stop him,' said Archamus. 'Won't we, sir?'

'Of course we will. I just wish—'

'What?'

'Nothing.'

'You wish what, sir?' Archamus asked.

'Nothing.'

The wind suddenly pulled at Dorn's fur-edged robe. Above them, the shields went out and then test fired again.

'Can I ask you a question, sir?' asked Archamus.

'Of course.'

'Who are you really afraid of?'

Consider the question, Rogal Dorn. The first axiom of defence is to understand what you defend against. What are you afraid of? Who are you afraid of?

Dorn paced the halls of the Kath Mandau Precinct where the organs of the Adeptus Terra did their work. The Precinct, an entire city contained within the terraced compounds of the inner palace, never slept. Robed clerks and burnished servitors bustled along the broad concourses. Ministers and ambassadors conducted business beneath the kilometre-high roof of the Hegemon. The great mechanism of the Imperium whirred about him, its relentless function like a licking timepiece. This was what Unity had brought, this and the near measureless expanse of worlds and dominions that it guided and administered.

For two hundred years, the Emperor and his primarchs had fought to create the Imperium. They had waged the Great Crusade from star to star, to forge the empire of man, an epic undertaking they had all made without hesitation, because they believed, with utter conviction, in the bright destiny it would shape for their species. They had all believed. All of them.

What was he afraid of? Who was he afraid of? Angron? Not him. Dorn would split his head without compunction if they came face to face. Lorgar? Magnus? There had always been a foetid whiff of sorcery about those two, but Dorn felt nothing towards them he could describe as fear. Fulgrim? No. The Phoenician was a singular foe, but not an object of terror. Perturabo? Well now, their rivalry was old, the spiteful scrapping of two brothers who fought for a father's attention,

Dorn smiled despite his mood. His years of exchanged insults with Perturabo seemed almost comical compared to this. They were too much alike, too jealous of one another's oh-so-similar abilities. Dorn knew it was a weakness for him to have risen to the Iron Warrior's baiting. But competition had always been a motivating force amongst the primarch brothers. It had been encouraged as a factor to drive them on to greater and yet greater accomplishments.

No, he was not afraid of Perturabo.

Horus-Lupercal, then?

Dorn's aimless wanderings had taken him to the Investiary. In that broad space, an amphitheatre open to the night sky, statues of the twenty stood on ouslite plinths in a silent ring.

There was no one around. Even the Custodian Guard was absent. Lumen orbs glowed on black iron poles. The Investiary was two kilometres in diameter. Under the glittering stars, it felt like an arena, where twenty warriors had gathered to make their combat.

The second and eleventh plinths had been vacant for a long time. No one ever spoke of those two absent brothers. Their separate tragedies had seemed like aberrations. Had they, in fact, been warnings that no one had heeded?

Sigismund had urged that the effigies of the traitors also be removed from the Investiary. He had offered to do the work himself. This, Dorn recalled, had made the Emperor laugh.

For the time being, the traitors had been shrouded. Their towering, draped forms seemed like phantoms in the blue darkness.