Ah, that’s a slippery creature to define. It is a beast with many heads and more tentacles which have wound their way throughout the Empire. One might summarize it as the new régime controlling the Empire after the Estates General.
Varas Solabius, Call to Arms; Chapter VII: "Departures"
The Estates General, a general meeting of all senior Magistrates in the Empire, were convene for the first time in nearly ten millennia on the eve of the 21st millennium. One the first day of the council, a group of powerful consuls from the core-world Augustclavii (“the Ninety Delegates”) stormed out of the Consular Senate Chambers in protest when their leader’s motion, demanding that the “oppressive” procedures (including set session length) and pre-prepared legal drafts be discarded, was ruled out of order . In the Platea Oliro Courtyard they swore an oath not to return until their demands were met. After a week-long stand-off, the Emperor gave-in to their demands and they were re-admitted.
Thus was the New Order born. The Ninety Delegates ultimately forced the Estates General into indefinite session, thus arrogating to the Consular Senate hitherto unknown power to legislate and set policy versus the traditional absolute rule of the Emperor. Civil war erupted when seven-hundred of the Magistrates present revolted against this coup, vowing to end the Estates General by any means necessary. Blood initially ran in the very chambers of the Estates then erupted into full interstellar war.
After fifty years of brutal warfare that saw entire planets eradicated and the ranks of the Magistracy and the Acolyte nobility alike greatly depleted, the Ninety Delegates had established the primacy of the New Order and what remained of the Seven Hundred were now but the scattered "Old Loyalists" existing on the outermost margins of Imperial life.
The most obvious tenets of the New Order that differentiate it from the Ancien Régime is their denial of The Disease (see Introduction), the downplaying of the Magistrates in favour of a more "democratic" governance of the Empire, and an extremely pacifist posture towards the alien nations.