The Celestial Orrery is a device located at the heart of the Necron Oruscar Dynasty's crownworld of Thanatos and that projects a web of holograms and necrodermis that represents every star in the galaxy. Its representation of the galaxy is not just a model, because the Orrery IS the galaxy in miniature in some unknown way.
Altering the Orrery will also physically change that region of the galaxy. The Orrery can be used to predict the strategic flow of coming events, or even alter them outright.
The Tomb World of Thanatos is a hollow planet, and hidden at its heart is one of the galaxy's greatest treasures -- the Celestial Orrery. Crafted by the artisans of the Oruscar Dynasty long before the onset of the War in Heaven, this web of hologram and living metal is beyond price for its artistic value alone.
Yet the Celestial Orrery is far more than mere decorative finery. The tiny pinpricks of glowing light suspended within the impossibly intricate matrix record the positions of every star in the Milky Way Galaxy. Snuff out one of these lights and its physical counterpart will go supernova long Terran millennia before its destined time, bringing fiery oblivion to all nearby worlds.
Such an act cannot be performed without consideration, however, as each star destroyed in this fashion upsets the fundamental forces of Creation, setting off a catastrophic chain reaction. Only with the further manipulation of the Celestial Orrery can these forces be returned to their proper balance, and this invariably taken many thousands of standard years of constant and precise micromanagement.
With so much potential power at their fingertips, it is well that the Royal Court of Thanatos is not given to maniacal displays. Rather, they see themselves as gardeners of Creation and dispassionately use the Orrery in a precise and sparing manner, pruning the galaxy only out of need to prevent it from becoming wild and overgrown.
Alas, this restraint is not something universally respected. Unending war rages across Thanatos' barren continents and in the skies above, as the legions and fleets of the Oruscar Dynasty strive to prevent the Celestial Orrery from falling into the incautious hands of aliens and other Necrons alike.
13th Black Crusade
During the 13th Black Crusade in 999.M41, the Necron Overlord Trazyn the Infinite travelled to Thanatos to make use of the Celestial Orrery after the Bell of Saint Gerstahl, an ancient Imperial Shrine World artefact intended to ring when the forces of Chaos at last threatened to overwhelm the galaxy, badly damaged the master program of his own home Tomb World of Solemnace.
Trazyn's welcome in the Oruscar Dynasty's halls was less than effusive -- in part due to a misunderstanding over the Oruscar Glyph of Dominance, which had gone missing during his prior visit. However, after bargains were struck and promises made, Trazyn was finally permitted access to the Orrery itself -- if under the watchful gaze of Oruscar Lychguards.
As he stepped into the whirling sphere of living metal and holographic light, Trazyn realised something was amiss. The intricate webs that formed the links of the Orrery were under-lit by a crimson stain. It pulsed beneath the weave of worlds like an infection, forcing its way to the surface.
Something was coming. Something that would change the shape of the galaxy. Thus far, it had gone unnoticed, but there was no hiding anything from the Celestial Orrery, for it was not merely a representation of the galaxy, but a perfect reflection of it.
Trazyn realised the Oruscar had known about the rising corruption for Terran centuries -- perhaps even millennia -- but had made no move to combat it. They could not, for inaction was the price of their custodianship. But Trazyn had no such restraint -- indeed, he acknowledged no master save his own amusement. He could act, if he chose. After untold millennia, there was some appeal in playing at selflessness...But where to begin? Where was the cause of the corruption?
Trazyn lost track of time studying the Orrery's pathways, searching for the wellspring of the galaxy's woes -- the source of the blight worming its way through the galaxy's heart. At last he found it, far to the galactic northwest, bordering the Eye of Terror.
In the Orrery, that world was catalogued as a string of trinary data, whose details Trazyn deliberately forgot. To the Imperium of Man, it was known as Cadia. Trazyn could not recall setting foot there, not in all his travels. To his understanding, it was a drab, grey world, of interest only to the bellicose. On the other hand, if the role of saviour grew tiring, Cadia would surely offer opportunities to expand his collection of historical personages and artefacts.
Through the Orrery, Trazyn learned that Cadia was home to the Cadian Pylons, the great Necron-built devices intended to help close off realspace from the Immaterium millions of standard years before during the War in Heaven.
Trazyn ultimately travelled to Cadia during Abaddon the Despoiler's great assault on the world, and provided the Imperial defenders with aid against the forces of Chaos. He taught the Archmagos Dominus Belisarius Cawl how to activate the Cadian Pylon network present on the planet and used elements from his own collection of historical individuals to provide reinforcements for the Imperial defenders of the Fortress World at key moments during the final battle.
Unfortunately, these would prove to be insufficient to the task, and Cadia ultimately fell to the tides of the 13th Black Crusade. But not before Trazyn made one new acquisition to his collection -- the Lord Castellan of Cadia, Ursarkar E. Creed.
- Codex: Necrons (5th Edition), pg. 15
- The Gathering Storm - Book One - Fall of Cadia (7th Edition), "A Dolorous Warning," pg. 20