"An ambition to save humanity is almost always a disguise for the desire to rule it."
- — Attributed to the Panpacific tyrant Narthan Dume, in the era of the Unification of Terra (M30)
The Imperium Secundus, known also as the Unremembered Empire, was the second stellar empire of Humanity created by the Ultramarines Primarch Roboute Guilliman during the Horus Heresy following the horrific events of the Battle of Calth and the Drop Site Massacre of Istvaan V. The light of the great navigational beacon of the Astronomican was extinguished across half the galaxy through the efforts of the Word Bearers Legion's maleficent summoning of the massive Warp Storm known as the Ruinstorm. Cut off from the rest of the Imperium and unable to firmly establish whether or not the Emperor of Mankind still lived, or if Terra and the Imperium of Man had fallen to the Traitors, Guilliman instituted the ultimate contingency plan -- to create a second Imperium in order to ensure the survival of the Emperor's dream of a united Mankind.
"Heresy comes in many forms. It can be blatant, like the one which now tears down the stars, but it can be subtle too, accidental. To make example, the building of a new Imperium when the old one is not yet pronounced dead. -- I am not building my own Imperium, Gantulga. I am preserving what's left of the original."
Following the horrific events of the Battle of Calth, the XIII Legion had been badly crippled and no longer presented a viable threat to Horus' plan to drive on Terra. Erebus had managed to complete his blasphemous ritual on Calth's surface, which summoned a Ruinstorm to the galaxy's Eastern Fringe -- a monstrous Warp Storm larger and more destructive than anything spacefaring humanity had witnessed since the days of the Age of Strife. It would split the void asunder, dividing the galaxy in two and rendering vast tracts of the Imperium of Man impassable for centuries. The Ruinstorm would also isolate and trap those Loyalist forces caught behind it like the Ultramarines, preventing them from coordinating their efforts and supporting one another as the Traitor Legions moved towards Terra. It would even prevent them from warning each other, for a time, of the Warmaster Horus' betrayal and the interstellar civil war that had begun to consume the Imperium. The Ruinstorm would leave Terra alone in the void, infinitely vulnerable to the approaching shadow of Horus.
Cut off from the Imperium, the Lord Macragge decided to secure the Five Hundred Worlds of the Realm of Ultramar. He decided to anchor what remained of the Imperium of Man on the Ultramarines' homeworld of Macragge. The XIII Legion had discovered the means to help navigate their voidships through the Warp without the beacon of the Astronomican through the use of recently discovered xenos-technology on the world of Sotha. The Ultramarines had also managed to maintain some security of transit, and the possibility of proper recomposition of their voidships when transitioning from the Warp into realspace. For all intents and purposes, the Five Hundred Worlds were the Imperium for the people of the galaxy trapped behind the barrier of the Ruinstorm. Everything was theoretical at that particular moment -- the rest of the Imperium, the security of Terra, the survival of the Emperor. Macragge was the only practical thing in existence. It was the only thing Guilliman and his Ultramarines knew they had, and at such time of extremity, it was the only foundation he knew he could depend on. Guilliman hesitated to declare the formation of the Imperium Secundus formally and publically, having admitted it to his closest confidences only. He had the strength in his conviction, and would not shrink from the more unedifying aspects of it. It wasn't fear that he was usurping the Emperor every bit as much as Horus. It was grief. Grief that his father, the Emperor, and Terra, and the grand dream of the Imperium were lost, and the only way for their civilisation to survive was to consolidate within the Five Hundred Worlds of Ultramar. It was a burden Guilliman never looked for, and it was made heavier by sorrow.
The Avenging Son refused to declare himself as Emperor, though his closest advisors suggested he at least declare himself as Regent of the Imperium Secundus. In truth, Guilliman hated the word, and the thought of ruling. Guilliman knew he could not both command and rule. He could not administer this new empire and be its figurehead. Guilliman's closest advisors suggested that no one else could be head of state. No one else could possibly be regent, for he was the last Loyalist Primarch. The last loyal son. The only loyal son. He was meant to become what he must, to invest himself as the rallying point of Imperium Secundus -- to be Imperial, and reveal his renewed strength, his resolve, his mettle and the glory, like a phoenix rising from the ashes. Despite their urgings, Guilliman was still reluctant. He could not build an empire and put himself on the throne, even if he were the only candidate. It smacked of hubris, of arrogance, of overweening pride and foul ambition. It smacked of Horus Lupercal.
Guilliman felt that if he declared himself Imperium Secundus' new ruler, it would diminish him in the eyes of those who yet respected him, and it would simply confirm the doubts of all those who did not -- taking advantage of this crisis and naming himself king. They would say that the Lord Macragge displayed unseemly eagerness, jumping in to take unwholesome advantage of the situation. Guilliman would have taken any loyal Primarch left, even the barbaric Leman Russ, to serve as the acting ruler of the Imperium Secundus rather than himself. It was a burden Guilliman had no desire to take up, but ultimately he was left with no one else as a potential candidate, and it proved a burden he was finally willing to shoulder. But soon the answer to Guilliman's prayers were answered, in the form of the arrival of an Imperial Expeditionary Fleet translating in-system, newly arrived from the Thramas Sector in the Eastern Fringes. This new arrival was none other than Lion El'Jonson and his Dark Angels Legion.
Guilliman was overjoyed that one of his wayward brothers had arrived during his hour of need, but of all his brothers, he wondered why it had to be the Lion who found his way through the storm. Though the Lord Macragge greatly admired the lord of the I Legion, he felt there had always a shadow upon him. The Lion dwelled in secrets, and played his cards too close, for he walked by himself when he pleased. There was too much of the wild forest in him. The Lion should have been noble as any of Guilliman’s brother Primarchs, but they had never been close, and there was too much about him that was sly. Yet, despite his personal misgivings, Guilliman welcomed the Lion with the formality and respect due his prestigious position. The galaxy contained only eighteen Primarchs. The conjunction of even two of them was a singular moment when the balance of the cosmos was temporarily and specifically weighed, especially when those two were, perhaps, the most feted and respected war leaders of all. Guilliman offered his brother an honour guard of almost forty-seven thousand fighting souls, not to mention the million or more civilians crowded into the streets of Macragge for a glimpse of the Lion-lord and his famous warriors. The people of Macragge came to celebrate the arrival of the Lion.
Following a formal inspection of the Ultramarines Leigon's formidable Macragge Civitas, the capital city of the world, the two Primarchs retired to a private chamber to speak candidly to one another. It was here that Guilliman had created a meeting place for the Emperor and his twenty sons. A long table, carved from stone, dominated the length of the room. Around it were twenty-one chairs, all built for the giant scale of a Primarch. Each one was cut from the same mountain granite as the table. The chair backs were draped with banners, ach representing the personal symbol of one of the Primarchs and his Legion. The great seal, at the head of the long table, was draped with the Imperial pennant of Terra. Two of the other pennants were plain and made of bleached, un-dyed cloth, to mark a place of honour for the missing but never forgotten Lost Primarchs of the II and XI Legions. The other eighteen were the banners of the Legiones Astartes. Guilliman had created this chamber in the greatest city of his homeworld in the hope that someday, after the Great Crusade was over, that all of his brothers, all of them, could sit at a table with their father, as equals, and talk of the matters of empire.
It was within this private chamber that Guilliman revealed his plans to the Lion for the continuity of the Imperium. That it must endure, in one form or another. That they must make it endure. His brother had confirmed to the Lion that he was in the process of establishing a second Imperium upon the corpse of the first. Guilliman did not pretend to hide his motives, but he was only establishing such an empire in order to keep the flame of their faither's dream alive. This was not about empire-building, or thrusting for the main prize. The Avenging Son already had an empire – Ultramar – the Five Hundred Worlds. Guilliman's only intentions were to ensure the survival of their father’s dream. Terra might have fallen, and their father might already be dead. Whatever the facts, the Ruinstorm prevented them from knowing the truth. Guilliman was utilising the moment to move to his advantage, and not to use the crisis as an opportunity to usurp the rightful ruler of the Imperium. He was not Lupercal. Guilliman explained to his brother Primarch that the difference between himself and Horus Lupercal was that he did not want to be the new Emperor. The Lord Macragge proposed that the Lion take up the mantle as the new Regent of the Imperium Secundus rather than himself. But in the end, with the arrival of their brother Sanguinius and his Blood Angels Legion from the Signus Cluster, Guilliman convinced his reluctant brother-Primarch to rule as Regent of Imperium Secundus, for he possessed more of the noble aspects of the Emperor within him.
- Unremembered Empire (Novel) by Dan Abnett