Context: First section basically

‘The Orrery is a device capable of altering the alignment of the Archways, to alter the connections between the Hollow Worlds,’ said Lord Cheng. His voice had taken a hushed, almost resentful tone, as if he hated to speak out loud the secrets of his office, even to superior authority and with dire necessity. ‘It hasn’t been used for millennia, since the calamity which ended with Threshold being lost to the other Hollow Worlds. Accounts of that distant time, confused as they are, tell of great catastrophes from attempting to realign the worlds, of tides that drowned cities and suns that darkened for centuries.’

‘If this Orrery is so dangerous, why not destroy it?’ growled Haakan. ‘To leave such an advantage open to the enemy is foolishness.’

‘It has been tried,’ said Cheng. ‘But the Orrery is invulnerable, self-repairing any damage caused to it.'

Red Corsairs reach the Orrery:

It was an orrery in the strictest sense, a working representation of the Hollow Worlds, but it was neither a physical model with metal spheres moving on some visible mechanism, nor a hololithic display with the worlds projected in light.

No, this was something else. The cavern within the mountain was a vast space with a curved ceiling high above, and beneath that ceiling floated the Orrery, a planetary system in miniature. It was unclear how it remained suspended in the air, but it was fed with energy from a central pit in the chamber, in which a glowing mass of energy swirled. Periodically, bolts of fierce, wild energy would crackle between the pit below and the spheres above.

This was no projection. The spheres appeared to be made of stone and some translucent material, so that the interior of each world with its small sun was visible from outside, and Rotaka could recognise the continents of the worlds he had already visited. The model of each of these suns flickered with flame, a fiery imitation of its larger equivalent.

The smallest of the spheres was larger than Kolsh, and they floated in a cloud of shimmering particles, clearly representing the Siren Clouds. Between the worlds ran strands of coruscating energy, linking sun to sun and Archway to Archway, all feeding from an ice-blue sphere in the centre. The spheres were moving, too, and while the ice-blue sphere remained at the centre of the system the rest of the Hollow Worlds did not orbit it in any conventional sense; instead the push and pull of the different energy streams caused them to weave around each other, the threads of power that bound them stretching as planets passed each other within the clouds.

Within this system, the positioning of the planets bore little resemblance to the interlocking of the Archways. Yes, Laghast and Plini were at one edge of the Siren Clouds, and Trincul at the other, but between them planets physically close in the rotation were completely disconnected, while others had Archways stretching between them even though they were far apart.

It was a machine, or a mechanism, Rotaka realised. Not just the Orrery, but the actual Hollow Worlds it imitated. An artificially created mechanism of unreadable purpose, one that might only pass for a planetary system because of its scale, and the life supported within it.

Space Wolf gets yeeted by Huron:

Haakan believed this for the last second of life, before his huge, armoured body collided with the sphere of Hacasta, smashing through the brass orb and unleashing the wild energies within.

There was a second of intense agony for the Wolf Lord as raw power surged through every cell of his body, then Haakan and the representation of Hacasta both exploded, particles of organic matter and fragments of burned ceramite scattering in all directions.

Hacasta gets destroyed:

Through the Archway, she could see that whatever tremor she felt had also struck Hacasta. The three Red Corsairs had tumbled out of the way, leaving Folkvar where he was, more firmly planted to the ground while forced on his knees.

And behind him – and by the Emperor, Anju could not believe what she was seeing – the world itself seemed to be shaking apart, chunks of the ground rising into the sky, the whole curve of Hacasta folding in on itself. While the ground nearest to the Archway appeared relatively stable, the horizon seemed to be bending away, swirling towards an artificial sun which had turned black – not the absence of light but something deeper, a hole in the world.

Only a short distance and a world away, Folkvar looked up, his eyes locking with hers. He nodded, in some silent acknowledgement, and Anju remembered what he had said in the cave, that she had survived the battle in the Valley of Blades, and that she would survive further. Now she was surviving again. She returned Folkvar’s nod.

Then he was torn away, as was the whole of Hacasta, the iridescence of the Archway consuming itself and the stone of that great arch collapsing, falling down, and Anju Badya had to scramble to her feet and run before she was crushed, but as she ran she was less scared of what might happen to her, and more consumed by what she had seen.

Was reading through the strange worlds thread today and decided to post this. The Orrery is described as self-repairing, which is characteristic of Necron tech, and whatever happens to the miniature planets happens to the actual ones too, so it's basically a tiny Celestial Orrery. The Hollow Worlds are basically habitable Dyson spheres connected by stone archways, and the lost planet, Exultance, was able to purge some of the Warp corruption from Huron when he went there. All of this seems pretty similar to Necron technology and what they could do, but I might just be overthinking it.