'Then why did the Emperor not use it well?'

'Because he was weak,' said Sejanus, leaning in close to Horus. 'Unlike you, he lacked the will to master it, and the gods of the warp do not take kindly to those who betray them. The Emperor had taken a measure of their power for himself, but they struck back at him.'

'How?'

'You will see. With the power he stole from them, he was too powerful for them to attack directly, but they had foreseen a measure of his plans and they struck at what he needed most to realise those plans.'

'The primarchs?'

'The primarchs,' agreed Sejanus, walking back down the length of walkway. Horus heard distant sirens blare and felt the air within the chamber become more agitated, as if a cold electric current whipped from molecule to molecule.

'What's going on?' he asked, as the sirens grew louder.

'Justice,' said Sejanus.

The reflective surfaces of the tanks lit up as an actinic blue light appeared above them, and Horus looked up to see a blob of dirty light swirling into existence just below the ceiling. Like a miniature galaxy, it hung suspended above the silver incubation tanks, growing larger with every passing second. A powerful wind tugged at Horus and he hung onto the railing as a shrieking howl issued from the spreading vortex above him.

'What is that?' he shouted, working his way along the railing towards the stairs. 'You know what it is, Horus,' said Sejanus.

'We have to get out of here.'

'It's too late for that,' said Sejanus, taking his arm in an iron grip.

'Take your hand off me, Sejanus,' warned Horus, 'or whatever your name is. I know you're not Sejanus, so you might as well stop pretending.'

Even as he spoke, he saw a group of armoured warriors rushing through the chamber's doorway towards them. There were six of them, each with the build of an Astartes, but without a suit of battle plate, they were less bulked out and gigantic. They wore fabulously ornate gold breastplates decorated with eagles and lightning bolts, and each wore a tall, peaked helm of bronze with a red, horsehair plume. Scarlet cloaks billowed behind them in the cyclone that swept through the chamber. Long spears with boltguns slung beneath long, crackling blades were aimed at him, and he instantly recognised the warriors for what they were - the Custodian Guard, the Praetorians of the Emperor himself.

'Halt, fiends and face thy judgement!' shouted the lead warrior, aiming his guardian spear at Horus's heart. Though the warrior wore an enclosing helm, Horus would have recognised his eyes and that voice anywhere.

'Valdor!' cried Horus. 'Constantin Valdor. It's me, it's Horus.'

'Be silent!' shouted Valdor. 'End this foul conjuration now!'

Horus looked up at the ceiling, feeling the power contained within that swirling maelstrom tugging at him like the call of a long lost friend. He forced its siren song from his mind, dropped to the floor of the chamber and took a step forward.

Pdpping blasts of light erupted from the Custodians' spears, and Horus was forced to his knees by the hammering impacts of their shells. The howling gale swallowed the noise of the shots, and Horus cried out, not with pain, but with the knowledge that fellow warriors of the Imperium had fired upon him.

More blasts struck him, tearing great chunks from his armour, but none was able to defeat its protection. The Custodians advanced in disciplined ranks, pouring their fire into him and keeping him pinned beneath its weight. Sejanus ducked behind the stairs, sparks and smoking chunks ripping from the metal as the explosive bolts tore through it.

Horus roared in anger and surged to his feet, all thoughts of restraint forgotten as he found himself at the centre of the deafening storm. A bolt clipped his gorget and almost spun him around, but it was not enough to stop him. He ripped the guardian spear from the nearest Custodian and smashed his skull to splinters with a single blow from his fist.

He reversed his grip on the spear and slashed the next Custodian from collarbone to groin, the two shorn halves swept up by the howling winds and vanishing into the crackling vortex. Another Custodian died as Horus rammed the spear through his chest and split him in two.

A blade lanced for his head, but he shattered it with a swipe of his fist and ripped the arm from his attacker with casual ease. Another Custodian died as Horus tore his head off in his mighty fist, blood gushing from the neck, as if from a geyser, as he tossed the severed head aside.

Only Valdor remained, and Horus snarled as he rounded on the Chief Custodian. A blaze of light erupted from the barrel of Valdor's guardian spear. Horus granted at the impacts and raised his fist to strike Valdor down, hearing metal squeal and tear as the force of the hurricane reaching from the vortex above finally achieved its goal.

Horus paused in his attack, suddenly terrified for the fate of those inside the tanks. He turned and saw one tank spewing gasses and screams as it was ripped from the ground, following others as they were torn from their moorings and swept upwards.

Then time stopped and a blinding light filled the chamber.

Horus felt warm honey flow through him, and he turned towards the source of the light: a shimmering golden giant of unimaginable majesty and beauty.

Horus dropped to his knees in rapture at the sight. Who would not strive to worship so perfect a being? Power and certainty flowed from the figure, the secret mystery of creation at his fingertips, the answers to any question that could be asked there for the knowing, and the wisdom to know how to use them.

He wore armour that gleamed a perfect gold, his features impossible to know, and his glory and power unmatched by any being in creation.

The golden warrior moved as though in slow motion, raising his hand to halt the madness of the vortex with a gesture. The maelstrom was silenced, the tumbling incubation tanks suspended in mid air.

The golden figure turned a puzzled gaze upon Horus.

'I know you?' he said, and Horus wept to hear such a perfect symphony of sound.

'Yes,' said Horus, unable to raise his voice above a whisper.

The giant cocked his head to one side and said, 'You would destroy my great works, but you will not succeed. I beg you, turn from this path or all will be lost.'

Horus reached out towards the golden warrior as he turned his sad gaze to the incubation tanks held motionless above him, weighing the consequences of future events in the blink of an eye.

Horus could see the decision in the figure's wondrous eyes and shouted, 'No!'

The figure turned from him and time snapped back into its prescribed stream.

The deafening howl of the warp-spawned wind returned with the force of a hurricane and Horus heard the screams of his brothers amid the metallic clanging of their incubation tanks.

'Father, no!' he yelled. 'You can't let this happen!'

The golden giant was walking away, leaving the carnage in his wake, uncaring of the lives he had wrought. Horus felt his hate swell bright and strong within his breast.

The power of the wind seized him in its grip and he let it take him, spinning him up into the air and Horus opened his arms as he was reunited once again with his brothers.

The abyss of the warp vortex yawned above him like a great eye of terror and madness.

He surrendered to its power and let it take him into its embrace.