A Daemon creates a solar system sized fortress

They encounter a Solar-System sized space fortress blocking their way. The fortress has its own gravitational pull that attracts them even though they are millions of miles away, and they quickly realize that the Fortress is impossible:

Spoiler:

The Invincible Reason plunged towards the wall of an impossible fortress. The Lion stared at the vision, and for several seconds his mind was unable to reconcile the structure with its size. It would have inspired awe had he seen it from the cockpit of a Thunderhawk. From the bridge of a ship, it beggared belief. He looked upon twisted, spiked battlements and towers of brass and iron. They rose from a wall that stretched to port and starboard as far as the Lion could see. The wall bristled with what, from this distance, looked like thorns and claws. The glow of ugly fires shone from innumerable apertures, a galaxy of pinprick flames. Light the colour of blood and hate moved over the fortifications, a nebula of horror.
The fortress filled the oculus, the wall dropping beyond the frame. There was nothing to see except the battlements, nothing to give the structure scale, but at last the Lion grasped its full monstrosity. The fortress spanned a system. The wall was tens of millions of miles high. It was billions of miles long. And though the proximity was lethal, it was still millions of miles away.
The fleets were caught in its gravitational well. They sailed across the void before it, pulled towards a collision, minute specks of dust blown at a mountainside.
‘Hard to starboard,’ Captain Stenius ordered. In his command throne one level below the Lion’s position, he leaned forwards, pulling at the mechadendrites that linked him to the Invincible Reason, as if he could lend the ship greater mobility by the actions of his body.
The Lion opened a fleet-wide vox-channel, and heard the same command echoing and re-echoing. It was too late to attempt to reverse away from the fortress. But the ships might yet avoid disaster by taking the momentum into a turn, moving parallel to the barrier using a slingshot manoeuvre to break the wall’s grip. The Lion shifted his attention back and forth between the monster growing larger in the oculus, and the trajectory screens for the combined fleets. The lines were shifting, slowly. The fortress was coming closer, quickly. The prow of the Reason shuddered before the Lion’s gaze, mile upon mile of gothic majesty wrenched by forces that reduced it to insignificance.
‘Hails from the Red Tear and the Samothrace,’ said the vox-officer.
‘Private channel,’ the Lion said. His brothers wished to speak to him. As he had expected they would.
‘Where have you brought us?’ Guilliman demanded.
‘I don’t know, Roboute.’
‘Neither do I. We can’t identify the system, whatever it once was.’
‘If this is a system, it has been moved,’ Sanguinius said. ‘But the location is not as important as its nature. This is not the work of Horus. It cannot be the work of material hands, be they human or xenos.’


The Fortress has War-Horns the size of Gas Giants, which emit waves of sound that cross the millions of miles between it and the Primarchs' fleet nearly instantly. The laws of physics have little meaning inside the Ruinstorm:

Spoiler:

The Lion turned back to the oculus. The angle of the Invincible Reason’s approach had become oblique, though the fortress was so vast, its expanse stretched for an eternity into the void, glowing and pulsing with its unnatural fire. Conical shapes jutted out from the battlements at irregular intervals. They were the size of gas giants. They could not be what they appeared to be.
Sanguinius confirmed the madness.‘They’re war-horns,’ he said. His tone was flat, and leaden with prophecy.
The horns blared a challenge. Across the airless void, they made their terrible, warp-created sound. It travelled the millions of miles that separated the fleet from the fortress. Perhaps the cry had come as soon as the fleets had been detected and it was only now reaching the ships. Perhaps the horns had only sounded on the instant. The laws of reality had been suspended in this star system, and the Lion knew that what mattered was not how the horns cried, but that they did. The sound smashed into the Invincible Reason, shaking the hull. It boomed through the bridge. It was deep, as deep as the heartbeat of mountains. And it was a wail, a rising, shrieking, raging wail that harrowed the soul. The Lion winced. He made himself breathe through the blast. He leaned into it. On the bridge below, officers screamed, blood running from their ears and eyes. Servitors collapsed, spines ground to powder. The electrical systems of the ship surged and stuttered, blowing out pict screens, setting control stations ablaze. The thrum of the battleship’s engines became a hammering roar, yet the cry of the horns sounded above everything.


In desperation, the Primarchs command the entire fleet to shoot at the Fortress, hoping they can burst a path through it. They vaporize enough metal to create a crater as wide of the whole fleet and four thousand miles deep. It does nothing:

Spoiler:

‘Do any of us think we can destroy it?’ the Lion asked.
‘We are all agreed that we must try,’ said Guilliman.
The greatest single naval barrage in human history occurred less than an hour later. Hulls vibrating from the strain of engines pushing back against the pull of the fortress, the formation closed to firing range with the construct. The wall filled the oculus of the Invincible Reason completely. The Lion could see nothing but the iron, the brass, the flames and the thorns resolving themselves into guns taller than Olympus Mons.
The weapons systems of the Imperium powered up. When they were ready, when the synchronisation of fire was arranged, when the speed of torpedoes was calculated against the immediacy of lances, so that every hit would strike the wall at the same moment, then the Lion, in concert with his brothers, said, ‘Fire.’
Fire.
The fire came to burn the void. More than a hundred ships opened up with every weapon. Macro-cannon batteries, ranks of lances, nova cannons, cyclonic torpedoes and more unleashed the anger of humanity against the obscenity before them. The raging of the Ruinstorm faded before the searing light of purest, purging destruction. It was an act of war on a scale that had never been witnessed before. If there had been remembrancers aboard any of the vessels, they would have felt compelled to record an event so monumental in song and in verse.
The barrage struck the fortress, and then it did not matter that there were no remembrancers. The action would not be remembered. There would be no songs. The immense became the insignificant. The explosions that erupted on the face of the battlements lit up the view in the Invincible Reason’s oculus. But the Lion made the mental adjustment, and understood how tiny the site of the impact was in relation to the wall as a whole. It might as well have been invisible, a momentary glint on the brass. We aren’t trying to destroy the barrier, he reminded himself. We need to pass through it. That is all.
The flare of the blasts faded. Geysers of molten metal extended into the void. Burning gas dissipated. A crater as wide as the fleet appeared. It glowed from the heat of its creation.
‘Our auspex readings put the depth of the breach at approximately four thousand miles,’ Guilliman said.
‘It might as well be nothing at all,’ the Lion muttered, disgusted. The crater was a meaningless blemish on the barrier. The wall could be millions of miles thick. There was no return fire. The fleets did not even register as a threat for the things inside the fortifications.


They later go through the Fortress' gate in search of its foundations, and they find a tiny planet inside the system-sized Fortress which is serving as a nexus of Empyrean energies:

Spoiler:

There was a gate. It came into view as the fleets travelled towards the galactic east. As far as the Lion could tell, it ran the entire height of the wall. By this time, the peaks of the battlements were long out of sight. The gate extended above and below the Invincible Reason’s position to the limits of perception, a universe of daemonic might. The fleets increased the angle of descent, heading for the promised base.
For much of the journey, the gate seemed endless, but it had a design, and so the Lion knew it must end. It took a long time for him to make out the engraving in the middle of the portal. This close, it resembled a mountain chain, a topography as big as a thousand worlds pressed together. The mountains had lines, though. They were constructed, they were art, and so they had meaning. The Lion watched the portions of the design as it passed by the oculus. He held the fragmented visions in his head, and assembled the pieces into a whole. It was an eight-pointed star. It was a brand upon the universe. It was a wound in reality, and it was a declaration of rule. As soon as the Lion understood its contours, it seemed to look back at him, an eye more monstrous and knowing than any of the red glows in the wall. It gazed at the dust motes of the fleet, and it looked upon the Lion from inside his mind.
His lip curled in defiance. He focused on the progress of the Reason’s journey. He promised the symbol destruction as he stared forwards. We will hurl you from the galaxy. By whatever means necessary.
The gate and the star slid by. The pace was agonising, though the fleet moved at full speed. The heat bloom detected by the Blood Angels became more intense and more defined in the long-range auspex scans. At last, its source became visible.
‘Sanguinius,’ the Lion voxed, ‘we have located your foundations.’
The eastern hinge of the gate was anchored to a world. As it came into sight, the planet was tiny, barely an excrescence at the bottom of the wall. But the intersection of gate and planet flared with enormous energy, far more than the disparity in size would suggest. The bloom originated in the planet, and travelled, spreading out a long way up the height and breadth of the gate before it faded.
‘So pitiful a foundation for so immense a construct,’ said the Lion.


As they approach the planet, they discover more about it. It is identified as the planet of Pyrrhan, and in it stands a mountain-sized Manufactorum Forge from which the Warp-energies are flowing:

Spoiler:

Sanguinius walked across the Red Tear’s bridge, taking in the portrait of the planet emerging from the combined rapid scans of the fleet as they developed on the arrays of auspex screens. It was a small, rocky world, not much larger than a planetoid, less than three thousand miles in diameter. Steep mountain chains dominated much of the south, diminishing as they approached the equator. Tens of thousands of years of intensive mining in the once-inhabited northern hemisphere had worn the peaks down. The composite images of the north showed the traces of human civilisation and industry, now twisted almost past the point of recognition. Manufactoria taller than the mountains they had replaced belched fire and smoke into the world’s eternal night. A spiral pillar, five hundred miles wide, linked the north magnetic pole of Pyrrhan to the bottom of the gate. It emerged from the greatest manufactorum, a jagged monster that rose from the surrounding terrain like two gigantic, clawed hands clasped in prayer. There was no clear division between manufactorum and pillar. They flowed into one another, and the pillar flowed into the gate. The pillar shone with the light of molten matter.
Sanguinius took what he saw with him to the hololith chamber. ‘Matter,’ he said to the images of his brothers. ‘Matter is being created at this juncture. We can observe it happening. The pillar grows from the manufactorum. The gate’s being is forged here.’
‘It cannot be the planet’s own matter,’ the Lion said. ‘It is a speck compared to the gate. It would have been consumed a million times over by now.’
‘There is nothing about what we are seeing that is rational,’ said Guilliman. His jaw was set with frustration. He said rational with something like bereavement. ‘This fortress is already impossible a million times over.’
‘Yet Pyrrhan is somehow necessary to it,’ Sanguinius pointed out. ‘The impossible has attached itself to something real.’
Guilliman’s eyes widened in the hope of reason. ‘An anchor,’ he said. ‘The fortress needs to anchor itself in the materium. Maybe Pyrrhan’s industry, in the new configuration, is a gateway, funnelling in the warp’s substance, making it into stable matter.’
‘That conforms to what we are seeing,’ said Sanguinius. ‘The intensity of the energy suggests an important nexus of some sort. Brothers, I would thrust a sword into that heart and sever the connection.’
Guilliman nodded. ‘Destroy the forge. Shut down the flow of matter. Perhaps that might destabilise the gate.’


And so the Primarchs decide to invade the planet with their Legions to destroy the Forge, in hopes of destroying the Fortress. Naturally, both the Fortress' defenses and legions of Daemons try to stop them. After a long battle, they arrive on the Manufactorum. There, they feel a tremendous psychic power inside, welcoming them:

Spoiler:

The IX Legion moved at a brutal run between the doors, racing to get through before they shut with crushing finality. There was a mile of open floor between the doors and the tunnel mouths. The surface was metal, though it looked like knotted flesh. With the Sanguinary Guard as point again, Sanguinius crossed the threshold in the manufactorum and directed the phalanxes to the left and right.
‘We take the widest halls,’ he ordered. ‘One company in each path. Captains, choose your terrain to fight upon.’
‘What are we looking for?’ Raldoron asked.
‘We’ll know when we find it,’ Sanguinius answered. ‘Believe that this is still a manufactorum. We can destroy its ability to function without having to destroy the structure itself. Go now, for Baal, for Terra and the Emperor!’
He was running forwards as he spoke. His army was spreading out behind him, escaping the doors. He chose the central hall for the Sanguinary Guard and First Company. Halfway across the floor of the towering vestibule, Sanguinius looked back. The doors were almost closed. The last of the Blood Angels were inside. He could just see a narrow slit, flickering with the flames of the battle outside the manufactorum. ‘Brothers,’ he voxed Guilliman and the Lion, ‘we are inside.’
‘Well fought,’ said the Lion. ‘We will scrape the land clear of the abominations for your extraction.’
‘Does the interior suggest how you can shut the forge down?’ Guilliman asked. ‘I would think–’
Vox contact with the exterior of the manufactorum cut off. The doors closed with a tectonic boom.
As Sanguinius led the march to the central tunnel, to where daemons gibbered and snarled and chanted, a psychic shudder rippled through the immense forge. It passed over Sanguinius. It was the touch of malevolent sentience. It felt like a welcome.


They walk through the Manufactorum's interior. The Forge is a living entity, with warp-energy flowing through arterial conduits.

Spoiler:

The manufactorum breathed. It was metal and stone, a monstrosity constructed on a framework of brass and iron girders the size of cities that gave shape to the bones of the planet. It was a work built in the materium, of the materium, and yet it breathed. The taint of the warp oozed from every crack in the metal, every pore of stone. The floor beneath Sanguinius’ boots heaved up and down with the slow inhale and exhale of lungs. It slithered too. Footing was treacherous. The metal felt as if it were covered by a thick, viscous slick of ichor. It was not. It was dry. Serpentine tremors moved through it, shifting and pulling, the surface changing from rough to smooth in the space of an instant.
The manufactorum bled. Where bolt shells or daemon claws struck the walls, rotting blood ran. The wounds puckered and sucked like hungry mouths.
And the manufactorum sang. The conduits were the pipes of an organ. Whatever moved through them summoned notes of cancerous music. The deeper the Blood Angels ventured into the forge, the more complex the song became. The mouths of open conduits had teeth, and tongues lapped at their edges. They shaped themselves to the demands of the tune. The tongues vibrated. The octaves of the sick melody plunged so far beneath hearing, the vibrations would have shattered a mortal’s skeleton. They rose so high, Sanguinius tasted blood. The music was a polyphonic symphony of dark industry and darker intent. It was a hymn to ruin, and a threnody for hope. In the complex nodes that developed, he thought he heard the formation of a whisper. It was at the edge of perception, unwilling to declare itself yet. There were no syllables. It was the hesitation that came before a familiar word was pronounced.
Sanguinius was not alone in hearing the whisper. Raldoron spoke to him when First Company and the Sanguinary Guard broke through another horde of the corpulent, plague-bearing abominations. ‘Something is speaking here.’
‘And it wants us to listen,’ Sanguinius said. ‘Shut your ears to it. There will only be poison in the words.’


Sanguinius and his Legion reach the center of the Forge, and inside it they find the source of the psychic power that was welcoming them: A single Daemon, the one responsible for the Fortress' creation. As Sanguinius approaches to face him, he is hit with a blinding light of Warp-energy:

Spoiler:

At last the corridor narrowed and made a sharp turn to the right. It ended in another nexus chamber. In the chamber was a single abomination; it stood before the junction of conduits. It was kin to the one Sanguinius and the herald had fought outside the manufactorum. It was taller, its horns longer, but the muscle-pink of its hide was the same. It carried a staff instead of a sword, and this it held at the vertical. Its other two hands held a book. Sanguinius did not know if it was the same tome or not. He didn’t care. The daemon appeared to want him to care. It held the book open. It held it forwards, arms extended, an invitation Sanguinius would decline.
A nimbus of warp energy surrounded the daemon. It dissipated as the Blood Angels came into view. As the energy vanished, the sounds of the daemon army fell into silence. The illusion of the multitude evaporated.
‘So this is our ambush,’ said Azkaellon. ‘A single foe.’ He sounded very suspicious.
‘Kano?’ Sanguinius asked. He advanced slowly into the chamber. Several hundred yards separated him from the daemon. He saw no other abominations lying in wait.
‘I sense only the one we see,’ the Librarian voxed.
Sanguinius hesitated. The daemon was motionless. It did not attack. It held the book, and that was all. It had lured him here. It must know what he planned to do. It could not think it would be enough to stop the entire company.
His choices were few. He had come to destroy the nexus. That imperative had not changed.
Sanguinius halted less than fifty feet into the chamber. He would not be part of the design being woven. ‘Fire on the nerve cluster from this position,’ Sanguinius commanded. ‘Ignore the daemon.’
The sorcerous light returned, and a sudden nova burst from the book. It shot up and behind the daemon, striking the nexus.
‘Back!’ Sanguinius ordered.
The conduits disintegrated. Coruscating, murderous light burst from them. It formed a contained ball of energy, blinding in intensity, its convulsions scraping at the mind like claws. It fused tighter, drawing in on itself. It imploded, and now, where the junction had been, was a single point of warp light.
Oh, Sanguinius thought, and then the light came for him.


Sanguinius and the Daemon have an incredible battle, where the Daemon attacks him on a physical, mental, and spiritual. Through sheer strength and will, Sanguinius is victorious. I will post the entirety of the fight here, because it is just too much awesome not too:

Spoiler:

There was a moment of absolute void and then there was a maelstrom of creation, an uncontrolled explosion of being without form. He dropped down its eye, into dark. He fell through blackness veined with red and green and blue and violet, the colours of ruin entwined and cutting into his mind like wire.

He did not land, but he stopped falling. He floated in the black. The darkness became mist, gathered definition, and became the bridge of the Vengeful Spirit. He was there again, as present as he had been when he had slipped between realities on the Red Tear. And again, Horus struck him down.

The pain again. The pain of final dissolution. He felt himself die, and he felt the grief at treachery and the grief of failure, the grief that his death was meaningless. There was no doubt now. He died, and Horus won, and that was all.
He fell into the dark, and light pierced it again; it became shadows and then substance again, and once more Horus towered over him. Once more, Horus slew him.
The pain, the grief, the futility… It was all there again.
And then again.
And again.
And again.
He lived his final moment over and over. The pain never diminished. Each iteration of his doom was as excruciating as the last. He tried to turn his head as the Vengeful Spirit materialised. He was confronted with the same perspective. He groaned, and with a wrenching effort of will, he turned his back on Horus. He spun himself away from the vision, though he heard the sound of his boots against the deck.'
The effort was futile. He turned and turned and turned, and he died and he died and he died. There was no escape, no meaning, no hope. Each time Horus killed him, he saw a trace of the future that would come. Every death brought its consequences. None were repeated. All were terrible. Horus was endlessly triumphant. All that changed was his immediate route to power.
Sanguinius spun faster, the movement no longer of his volition, and he saw endless iterations of his fate at once, as if he were in a mirror corridor, with the same image reflected to infinity. But this was more than an image. It was real. He died a thousand times at once, and the pain was a thousand times amplified.
A new blackness rose. This one was his. It came from within. It was anger that dwarfed the pain. Its jaws were immense, jagged with fangs, breathing flame and hatred. It consumed him, and yet the jaws were his too. With them, he would repay the treacherous galaxy with blood. He roared in an agony of wrath as the images of his death endlessly repeated. Darkness within lashed at the darkness without. Deep in his mind, buried in the agony, a sliver of his identity protested. It did not want this thing he had become. It rejected the black monster that claimed him as its father. His last piece of rationality reached out in desperation for any trace of hope. It flashed through the deaths upon deaths upon deaths, becoming weaker and dimmer as it looked for any meaning, any sign of doom leading to consequences other than eternal night for the Imperium.
At last, the hope appeared. In the infinite variations of the battle against Horus, there was one that challenged predestined death. One vision was different. Only one. In it, Sanguinius dodged the death blow. Horus put so much power into it that he overbalanced when he missed. There was a fraction of a second of hesitation as he sought to correct his momentum. Sanguinius seized the chance, and drove the Blade Encarmine up. It found the weakened point in Horus’ gorget. It pierced his armour.
It pierced his throat.
It pierced his skull.
This was the outcome Sanguinius had never even dreamed of, because the entire foundations of his destiny were built upon its denial. 'In this one vision, the terrible blackness withered, collapsed and broke apart into whirling ash. Here, and only here, there was hope. Here, and only here, the Imperium did not fall. Here, and only here, the Emperor’s dream lived on.
Like the coils of a monstrous serpent, the continuum of dooms gathered around Sanguinius. They came to strangle the single hope. They came to destroy the thing that turned them from certainties into mere possibilities. The visions formed a knot around Sanguinius and the image of light. It tightened. The wrenching anger rose once more, fastening its grip on his being. When the knot closed, the light would be gone, and there would only be the fury.
Sanguinius felt the imperative of decision. He made his choice. He refused the blackness and chose hope. He lunged towards the light. He raised his sword and plunged into the vision. He became the Sanguinius who slew Horus. The sword killed Horus, and it cut through his evil dream. It sliced through the knot. The visions fractured, mirrors broken into shards. They fell away into the void.
Now Sanguinius was falling again, but he was not done with the sword. After driving the Blade Encarmine through Horus’ skull and through the knot of fate, he brought it down. There was so much strength in the blow, it cut through unreality itself.
The blade sank into the flesh of the dream void. It tore the warp fabric open with fire. Light appeared beneath Sanguinius as he fell. He was leaving the realm of visions, but had not yet returned to reality. He was in a nether zone, neither dream nor real, but linked to both states. The light was another knot, another nexus. It looked like the intersection of the manufactorum conduits, only the conduits themselves were absent. It was the flow of warp energy and material mined from the interior of Pyrrhan, the unreal colliding with the solid to form the gate. Searing, impossible creation whirled through the convulsions of the nexus. It was the forge of the new reality of the galaxy. Sanguinius descended, wings outspread, to cleave the knot in two.
He passed through the nexus. In his wake, creation ended. The energy screamed. The uncontrollable explosion spread though the void. A dome of conflagration unfurled above Sanguinius.
Still he fell, still powered by the single sword blow. Another knot of energy appeared, and he cut it, and then the next, and the next, and the next. He had chosen to live, and so had torn destiny open, and with that rupture was slicing apart all the coils with which Chaos sought to entangle him. He was dealing a death blow to a creature he barely understood, whose size he could not grasp, and yet had defeated.
Down, down, down, the great sword blow of hope cutting through all the knots of ruin, a streak of red unleashing the flames of purification, until he cut his way back to reality. The void peeled back. The light of deliverance engulfed him. It blinded him with the blood-red fire. He blinked, and the flare vanished. He stood in the chamber from which he had first fallen. The daemon was at his feet. Its body was cut in two. As was its book and its staff. Weapons and corpse curled at the edges, dissolving from the materium. Violent light filled the chamber. A sea of liquid fire roiled across the ceiling. Currents of flame strove against each other. Maelstroms formed, melting everything above. Metal and stone dropped in blinding streams to the floor of the chamber. Everything was shaking, cracking, breaking apart.


What he destroyed in the Warp is destroyed in the Material universe, and as a result of the Daemon and its power being slain, the whole Manufactorum begins to collapse:

Spoiler:

‘Lord Sanguinius?’ Azkaellon was at his side, half reaching as if unsure whether or not the Angel was an illusion.
Sanguinius looked around, trying to get his bearings. He felt the solidity of the floor beneath his boots. The surface quaked, but did not flow away into dream. He looked at his sons. The Blood Angels were more or less as he had last seen them. More of the company had entered the chamber, but that was the only change in their disposition he could see. They were solid, unwavering in their reality. ‘What happened?’ he asked.
‘You vanished,’ said Azkaellon, alarmed.
On the Red Tear, he had fought visions, but they had been visions. He had not fallen into an elsewhere. The fusion of dreams and reality made him dizzy. ‘How long was I gone?’
‘A few seconds. You and the daemon.’
It had felt like hours. A lifetime. As long as death.
His vox-bead vibrated, erupting with the reports and interrogatives of his captains and Chapter Masters. He listened to them now, putting together what had happened during his absence. He realised after a few moments that he was the author of the new situation in the forge. What he had cut apart in that netherzone was now destroyed in reality. Tangles of conduits all over the manufactorum had exploded, far more than had been targeted. The complex trembled. Its song was broken beyond repair. The manufactorum screamed now. It howled a chorus of final pain and terror. Machines and power were tearing each other apart.
In the hail of reports coming over the vox, a fragment broke through. It was a single word. ‘Brother.’ The rest was lost in static. But the voice was Guilliman’s. The Blood Angels were no longer isolated from the other two Legions.
The tremors were growing more violent by the second. The forge’s scream rose higher, losing coherence, becoming a thunder of catastrophe.
‘We have triumphed!’ Sanguinius voxed to the Legion. ‘The enemy’s works fall before us.’
At his nod, Azkaellon ordered, ‘All forces, withdraw immediately.’
Spear held high, he led the turn away from the chamber. The tunnel beyond roared. Liquid flame consumed its ceiling too. The conduits split, bleeding energy. Uncanny lightning forked across the width of the corridor. It burned glowing crevasses into the walls and floor.
The Blood Angels raced to keep their victory. The mountainous forge was about to fall in on itself.
We do not die here, Sanguinius thought. My end is not yet.
And then he wondered, Are you sure?
He wasn’t. And the uncertainty elated him.


The Legions evacuate in time, and as they enter their ships, the entire solar-system sized Fortress collapses:

Spoiler:

The end of the manufactorum arrived as the combined fleets came over the horizon, putting them once more in the line of fire of the keep’s guns. The surface of Pyrrhan flared white. Coronas of warp energy whipped up the pillar, growing vaster with every slashing coil. Pyrrhan imploded, vanishing in an instant, the tiny growth at the end of the pillar sucked into the climbing disintegration. Blasts the size of miniature suns rocked the pillar, and then the dissolution reached the gate itself.
The gun emplacements on the gate fired briefly. They did not destroy their targets before destruction came for them. On the bridge of the Red Tear, Sanguinius watched the holocaust. Warp-infused flames hundreds of thousands of miles high consumed the gate. The structure lost coherence. It disintegrated. Shrapnel as big as planets spun away from the main body. Solid matter millions of miles thick became vortices, hurling streams of incandescent rubble into the void. The trailing edge of one stream hit the Dark Angels cruiser Claustro, turning the vessel to burning gas. The greatest vortex appeared in the middle of the gate. The eight-pointed star spiralled into fragments. The breach spread wider with ferocious speed. The colossal structure consumed itself like burning parchment, the edges peeling away from the centre, ash that could smash a battleship to dust billowing to the galactic east and west. The sides of the ruined gate glowed, molten. Between them now was a passage millions of miles wide. The void, stained by the Ruinstorm, was visible on the other side.
Engines powering up to maximum speed, the fleet began the long crossing between walls of infinite fire.

-Ruinstorm novel