Among all the warriors of the Empire, only the most upstanding of the Humble Order of Paladins were chosen for this honor, those who throughout their centuries long career showed upstanding moral character, unremitting devotion to the Empire and to the Greater Cause, and, of course, exceptional ability. The Medicants of the Order Transformarum would feed them draughts that would over the years boost their reflexes, speed up their minds, but slowly enfeeble their bodies. At the end of the decades long process, only the most promising would be taken up and be Measured, a mysterious process shrouded in secrecy. Those found Wanting would be Cast down and destroyed. Those few found Worthy would be Ascended into the Astral Plane, and would share in the glory of seeing the true face of Empress Celestia herself as Aurors.
In their new astral bodies, the Aurors would experience the passing of every second in the Prime Material World as if it were an era, and see regular humans as near-frozen in time. So all contact with the Old Life would cease, but this would be a worthy sacrifice, for who but these Most Upright could be trusted with the Challenge of Control of the Inexpugnable Defenses that lined the (ever-expanding) Outer Rim, and shielded the Empire from the Demonic Hordes that sought to bring it down? The Enslaved Machine Daemons controlling the massive Battle Platforms the countless Sensor Imp sentinels and vast Drone armies with their reality ripping weapons would answer only to the Aurorial Inquisition, who would carefully sniff out any trace of insubordination or betrayal, and occasionally put Daemons to the Test, to see if they were likely to fall victim to Pride and rebel against the Empire. The fragile worlds of the Empire were thus made safe for the Greater Cause.
A fitting punishment for disobedience, unrelenting pride, or insufficient devotion to the Greater Cause, the Casting is the most severe punishment in the Empire. These criminals have proven unworthy of the gifts that they received, so they are Cast down into the fiery pits, their minds laid bare and dissected, parts are cut out and others added. These unfortunate souls are in constant and unrelenting pain. Their astral bodies are incarnated again and again into Drones and enslaved to the Greater Machine Demons. Their only source of relief from their agony is a glorious death in battle against the Empire's manyfold foes, which they are glad to partake in.
Occasionally, though, the House of Infallible Justice has been known to err, and a innocent victim is rendered into the Casting. The Casters can see inside the victim's mind as if it were an open book, and so recognize the Erroneous Verdict, and refrain from much of the cutting that they normally do, and the Astral Mind of such a victim is mostly left intact. Their Drone incarnations are free of pain, and if they show Valor and Skill in close combat with the Enemies, can indeed rise to Drone Command, a field command similar in power to that of the Enslaved Machines. Some can even become Aurors with time. (Needless to say, this would be a good candidate for a main character)