He turned from the window and poured himself a cup of wine from a brass pitcher he lifted from a nearby table. He drained the wine in a single swallow and poured another as a rapid knocking sounded at the chamber's entrance.

Horus looked up, his mood souring further as he saw Fulgrim standing in the doorway, a gilt inlaid box held before him.

Once they had shared a brotherhood as close as any, but in the years since they had fought together, something had changed within Fulgrim. His brother had been a warrior of perfection, but now he simply revelled in the sensations of battle and the adrenaline high of ferocious combat instead of the precise application of force.

His brother wore his battle armour, the plates gleaming and new once again, as though he had never set foot upon a battlefield. He wore a long cape of fiery golden scales at his shoulders, and a mail shirt of glittering silver hung beneath his breastplate. What had once been a magnificent, all-enclosing suit of armour now resembled a theatrical costume. 'Warmaster,' said Fulgrim.

Horus detected a subtle difference in his brother's tone, something so slight that it would have escaped anyone else's notice but his. He lifted his cup and drank a mouthful of wine, beckoning Fulgrim into his chambers.

'You requested a private audience with me, Fulgrim,' he said. 'What is so important that you could not tell me in front of our brothers?'

His brother smiled and bowed before opening the box he carried. 'My esteemed lord and master of Isstvan, I have brought you a trophy.'

Fulgrim reached into the box and withdrew a grisly prize lifted from the field of battle. Horus felt a momentary shiver of horror as he saw the severed head of Ferrus Manus.

The flesh was grey and dead, his erstwhile brother's silver eyes plucked from his head, and the sockets raw and bloody. His jaw hung open and a splintered nub of bone projected from where his skull had been caved in on one side.

Ferrus had become an enemy, but to see his flesh violated so brutally was repugnant to Horus, though he was careful to keep his feelings veiled.

With a casual flick of the wrist, Fulgrim tossed the bloodied object at Horus's feet. Ferrus Manus's head rolled across the black floor and came to rest with the ravaged eye sockets staring up at Horus in blind accusation.

Horus looked up from the head and turned his gaze on Fulgrim, seeing again the insouciance that had infuriated him so when his brother had returned in failure from his attempt to win over the Primarch of the Iron Hands.

As distasteful as it was, he knew he would have to offer congratulations. 'Well done, Fulgrim. You have slain one of our greatest foes as you said you would, but I fail to see why you make this presentation in so private an audience. Surely you would wish our brothers to revel in your triumph?'

Fulgrim laughed, but there was a timbre to his brother's amusement that sent a chill down Horus's spine as he recalled where he had heard such ancient malice before… in the voice of Sarr'Kell, the entity Erebus had summoned in the heart of the Vengeful Spirit.

'Fulgrim?' asked the Warmaster. 'Explain yourself.'

The Primarch of the Emperor's Children shook his head and wagged his finger at Horus. 'With the greatest respect, mighty Horus, you do not address Fulgrim any more.'

Horus looked into his brother's dark eyes, seeing beyond the arrogance and superiority to what lay within. Darkness filled his brother's core, an ancient darkness that had torn itself from the womb of a dying race with a bloody birth scream.

Its existence was as old as the heavens and as fresh as the dawn. Its life was immortal and its capacity for malice infinite.

'You are not Fulgrim,' he breathed, suddenly wary of this intruder in his midst.

'No,' agreed the thing with his brother's face.

'Then who are you?' demanded Horus. 'A spy? An assassin? If you are here to kill me then I warn you I am no weakling like Fulgrim. I will break you before you can lay a hand upon me!'

Fulgrim shrugged and tossed the box he carried onto the floor with a clatter. It landed next to Ferrus's severed head. Horus let the energised claws of his gauntlets slide out in warning.

'Perhaps you can defeat me,' said Fulgrim, crossing the room to pour himself a cup of wine, 'but I have no wish to test either of us in such a fruitless and wasteful trial of combat. On the contrary, I am here to pledge myself to your cause.'

Horus glanced towards Fulgrim's waist, and relaxed as he saw that this thing masquerading as his brother had come before him unarmed. Whatever its purpose in unveiling itself, it had not come with violence on its mind.

'You still have not answered my question,' said Horus. 'Who or what are you?'

Fulgrim smiled and licked his lips with a long sweep of his tongue. 'Who am I? I should have thought that would be obvious to one who has had dealings with other creatures of my ilk.'

Once again, Horus felt the chill that he had experienced when the Lord of the Shadows had manifested in the stone-walled lodge, raised in the heart of his flagship.

'You are a creature of the warp?' he asked.

'I am indeed. What your insufficient language might call a "daemon". A poor word, but it will have to suffice. I am a humble servant of the Dark Prince, an emissary come to aid you in your little war.'

Horus felt his anger towards this impudent creature grow with every patronising syllable that dripped from its lips. It had usurped the body of one of his underlings, the fate of the galaxy was at stake, and it dared to call such a conflict ''little''!

The Fulgrim thing turned away from him and paced the length of his chambers, as though it had never seen a room quite like it. 'I have claimed this mortal shell as my own, and I must admit that it is most pleasing to me. The sensations one experiences when clothed in flesh are quite unique, though I daresay I shall have to make some alterations to its form in time.'

Horus felt his skin crawl at the idea of such a hideous violation. 'What of Fulgrim? Where is he?'

'Fear not,' laughed the warp creature. 'We have a long and… involved history, Fulgrim and I, and I certainly do not wish him any lasting ill. For some time I have been his conscience, quietly advising him in the lonely watches of the night, advising him, cajoling him, comforting him and steering his course of action.'

Horus watched as the daemon ran its hands along the sand-blown walls of the chamber, its eyes closing as it enjoyed the rough texture of the stone surface.

'Steering his course of action?' prompted Horus.

'Oh, yes!' exclaimed the warp creature. 'I made him believe that he should not doubt your course of action. Of course, he resisted, but I can be very persuasive.'

'You made Fulgrim join with me?'

'Of course! Did you really think you were that good an orator?' chuckled the daemon. 'You have me to thank for clouding his perceptions and adding his strength to yours. But for me, he would have run to his Emperor screaming of your imminent betrayal.'

'And you think I owe you something, is that it?' asked Horus.

'Not at all, for in the end, Fulgrim was weak, too weak to finish what his own desire had begun,' explained the creature. 'His obsession led him to launch the deathblow at his brother, but his weakness would not allow him to land it without my help. I merely gave him the strength to do what he wanted to do.'

'But where is he now?'

'I have already told you, Horus,' cautioned the daemon. 'Fulgrim's anguish at what he had done proved too great for him to bear. He begged me to help him extinguish his life, but I could not destroy him, that would have been far too prosaic. Instead, I gave him eternal peace, though not, I think, in the way he actually desired it.'