- It's Guilliman, What do you think he's doing? He's building an Empire! - Malcador the Sigillite
"Imperium Secundus" is the formal name for the secondary Imperium of Man, which Roboute Guilliman tried to build in the wake of the Horus Heresy around his own Legion Homeworld, the Ultramar systems. Data shows that Guilliman was preparing this new empire even before the Heresy, and many rumours state that he might have wanted to make an empire himself, instead of letting his father have all the fun. Others go that he had forseen the Heresy long before, and tried to do his best to keep humanity safe after the fall of their leader.
It was, and is, essentially skub in the form of a state. Ask yourself why the Ultramarine reinforcements were late for the Siege of Terra. Calth? The Ruinstorm? Ultramar being on the opposite side of the galaxy? Those plausible answers are only part of the reason. Instead, we get this... issue... to pad the Horus Heresy series.
Imperium Secundus (from the self referential book: "Unremembered Empire") was Roboute Guilliman's attempt at building his own Imperium of Man when he (incorrectly) believed that the Emperor lay defeated and that the rest of the Imperium was ashes during the Horus Heresy.
The Word Bearers' assault on Calth created a Ruinstorm which spread across the galaxy, garbling communications and restricting travel. The 500 worlds of the Ultramarines Legion had no means of determining whether the rest of the Imperium stood or not during this period of war.
Thus the analytical mind of the Primarch Guilliman resolved that according to the theoretical destruction of the rest of the Imperium, he would enact his practical solution and build his own.
As it required moving the power-base of his "new" Imperium and its institutions to Macragge, he knew that this situation could potentially be construed as an act of treachery. Thus, he maintained it would only be a temporary measure where if the Emperor and Terra were found to be intact he would be the first to lend his aid in their defense.
He also insisted that he would not be its ruler, as he was very aware that to both split from the true Imperium and to position himself as head of the splinter government would make him appear as a tyrant who was only interested in power. To prove his point, he planned to wait until such time as a worthy brother Primarch arrived on Macragge and make them the Regent-Emperor instead.
However, a number of events call into question Guilliman's sincerity:
Warsmith Barabas Dantioch of the Iron Warriors was lured into the service of Guilliman on the promise that his skills would be put to good use fortifying the Imperial Palace in the inevitable Siege of Terra; it seems Dantioch never got that far, and was assigned a position on Sotha, and possibly post-heresy his soldiers were just absorbed into the Ultramarines rather than making any meaningful contribution.
While this might sound cold and callous, there are the two following points to consider. First, the 500 worlds of ultramar were effectively cut-off from Terra by the Ruinstorm, so Dantioch might never have been able to get back. Second, as a Loyalist from a Traitor Legion, Dantioch and his men would have been mistrusted (if not outright imprisoned like the Outcast Dead or worse, executed); therefore, being absorbed into the Ultramarines would have been as wise a choice as any. It might not be the contribution Dantioch and his men wanted, but it definitely beats getting blam-med.
Mustering Forces to Himself
What Guilliman found on Sotha was a pre-Imperial device that acted similarly to the Astronomican. This device was known as the Pharos. Instead of a psychic lighthouse like the Astronomican, the Pharos shone like a lantern and illuminated targeted worlds in the Warp, allowing safe passage to them even through thick warp storms. Guilliman ordered that the Pharos be shone on Macragge so that lost loyalists fleets could find their way to him though the Ruinstorm.
This did have the intended effect, as beaten stragglers from the broken legions of the Iron Hands, Salamanders and Raven Guard found their way to Macragge. It also brought in those ships from the Imperial Fists Retribution Fleet that managed to retreat from Phall and some White Scars who were not at Chondax for some reason. Badly beaten, and with their own primarchs dead or assumed dead, they swore allegiance to Guilliman until such time as the Emperor could be aided.
Now this is where it gets ugly: both the Blood Angels and Imperial Fists were actually looking for Terra, so it could definitely be said that all these arrivals starved Terra of valuable resources and actually hindered the war effort. In fact this was explicitly stated to Lorgar by Kairos Fateweaver prior to the battle at Calth (the only time both of the damn bird's heas were compelled to speak the truth at the same time): they warned him that if Guilliman died, then Horus would have no chance at the Siege of Terra.
However, it is also known the Pharos was simply one part of a larger galactic network. It may be that Guilliman was incapable of redirecting its energies onto Terra, implying he was simply making the most practical use of the tools available. Not only that, but the Pharos was described as an "Empathic" device which responded to the desires of its user, but had mostly unknown capabilities. Thus, by shining it at Macragge, one could take from that that it responded to Macragge's (and by extension Guilliman's) desire to regroup and fortify. Which given the Ultramarines just got savagely attacked and mauled by Lorgar and Angron made sense, even if it had poor consequences for the Imperium at large.
Empire Building / Desire for Power
Guilliman was renowned for Empire-Building, and his attempt at usurping the Imperium was actually anticipated by both the Emperor and Malcador the Sigillite. They discuss the matter together and are thankful that Lion El'Jonson takes his own initiative to resolve the matter. Now the Lion may or may not have had his own agenda, as he had attempted to gain military power himself earlier in the war, but at least he argued that it was to the Imperium's benefit for him to be Warmaster and to be of better service to his father. The Lion was very clear on the issue with his brother Guilliman, he saw his brother's new Imperium as a threat equally as grave as Horus to the kingdom that his father had built- although he had no problem with Guilliman declaring him Warmaster of the Imperium Secundus in all but name.
Even Konrad Curze thought it was hilarious and internally mocked Guilliman in the "look at me, look at me, I'm building an Empire!" fashion. But then he was batshit insane to start with.
Guilliman was very politically minded and was careful not to take the position of Regent for himself, lest he be seen as a usurper of his father's Empire. So he resolved to ensure that the position would go to another Primarch as only a member of the blood would have the qualities to rule (just not himself). Unfortunately the first Primarch to arrive (Vulkan) had gone completely insane and thus was incapable of taking the position.
When the Lion arrived on Macragge, the issue of whether Guilliman should hand over rulership to him did come up. Unfortunately, Guilliman described the Lion as too inscrutable to be properly trusted with the position, and in turn the Lion could not trust Guilliman due to what the entire enterprise represented. (They never actually came to blows with one another over the matter, but nearly did since BOTH brothers were keeping secrets from one another, such as the locations of Konrad Curze & Vulkan). So, when Sanguinius and his fleet arrived, the two of them showed some common sense and unanimously decided to hand over the position of Regent to him. At the very least, both brothers could be described as sincere in their desire to put aside their differences and get shit done for the Imperium. Therefore, assigning the regency to Sanguinius was a matter of convenience for both parties involved.
Deathfire also shows that since Konrad Curze managed to singlehandely bypass all the security measures Guilliman put in place (along with disparate World Eater and Word Bearer raiding groups still plaguing Ultramar), he turned Macragge into a military fortress, complete with curfews, decreasing civil liberties, and the Ultramarines acting as police force. Those were the very things setting up Imperium Secundus was supposed to prevent, but to be fair to Guilliman it caused him considerable emotional distress.
Guilliman's intentions were apparently sincere, if not in their execution. The three Primarchs eventually found out that the Emperor was still alive due to a combination of Sanguinius and Kurze's precognition. Guilliman himself was utterly mortified at everything he had done based on the assumption the greater Imperium had fallen. Kurze, having spent a few years terrorising Macragge on the (very twisted) premise that what he was doing was what his father truly wanted, promptly pissed himself laughing from the irony of it all.
When challenged on the matter by Alexis Pollux of the Imperial Fists Retribution Fleet, Guilliman countermanded Rogal Dorn's order to return to Terra, citing that he held command unless someone who outranked him showed up (who could only be Dorn in Pollux's case or the Emperor himself) and that for all intents and purposes, Macragge already was the Imperium.
When Pollux then inquired if that made Guilliman the new Emperor, his reply boiled down to "I am the Imperium!". Guilliman's response to Pollux was a bit telling and bears the hallmarks of an excessively cautious man but not necessarily a traitorous one, particularly since it was well worded. By discounting the rest of the Imperium's existence as a mere theoretical and choosing to deal with only the tangible matters of the practical, it shows he was the sort of person who was unlikely to take risks and would prefer the path of certainty rather than gambling for something greater. Essentially Guilliman's solution to the Schrödinger's cat paradox is to get a new cat. By demanding proof of life he operates on the principle that the Imperium is dead until shown otherwise.
To be fair to Guilliman, it was two years before he starting receiving any news from the outer Imperium and when it did start coming in, it was mostly bad. And when he did get proof that the Emperor and the Imperium were still holding on, he stopped the whole thing immediately with no further resistance.
Oddly, Guilliman was willing to let the remaining Salamanders attempt to breach the Ruinstorm, despite having the power to stop them (or knowing that if they succeeded they could tell Terra what he was up to). It turned out to be moot anyway as even when they Salamanders found themselves in a position to warn both the Imperium AND return to Guilliman, they chose to do neither.
Foreknowledge / Intention
Guilliman was possessed of an analytical mind unlike any other, quite possibly greater than any of his brother Primarchs, and therefore was perfectly aware of what his new Imperium represented and did not need to be told by his brothers how heretical it was.
In his own mind Imperium Secundus was "Plan B" - an outcome less desirable, but something you can live with.
Despite this; Guilliman was fully prepared to fight for what he believed in and even related to his officer Remus Ventatus (in the same conversation when he iterated that the Codex Astartes should not be adhered to religiously) that he should even be prepared to fight against those brothers who counted themselves Loyal to the Emperor, and even later commented that when he was about to declare the Imperium Secundus official he was ready to kill anyone who would stand in his way.
Furthermore, the fact that the Codex Astartes was written well before the fall of the Emperor shows that it was obvious that Guilliman believed that HE was the only person capable of safeguarding his father's legacy, regardless about what his brothers might have thought in the aftermath of the Horus Heresy. This would only have been irrespective about who took the position of "Regent" (essentially making it a puppet role), since Guilliman planned it all well in advance and was willing to back it up with force.
...possibly rightly so, since the "apparent" aim of the Codex was to neuter the strength of individual warlords, followed up by the dissolution of the actual role of Emperor, all makes the distribution of power in a future Imperium ever so slightly more equitable and better able to uphold the ideals of the Emperor rather than ignoring them in favor of mindless loyalty towards the Emperor himself. Or at least, that's what his original plan was. Given how upset he was (and still is) about the state of the Imperium after he got out of his 9,000 year stay in stasis, it's almost certain that he was less than pleased with the end result.
Is this the sign of a Heretic? It could be interpreted as such, but considering that Guilliman was trapped on the far side of the Galaxy with no awareness of the full extent of what had occurred, helpless to cause any significant impact as the war progressed while his powerful predictive mind could calculate most likely probably outcomes of the war in his absence, one might call his actions prudent planning rather than underhanded scheming. Later events have confirmed this, as Guilliman was utterly shocked when he learned his entire rationale was based on a false assumption. In retrospect, his insistence on leading the Scouring was likely his way of trying to atone for his failure to return to Terra sooner.
On a final note, keep in mind Guilliman ultimately got manoeuvred into creating Imperium Secundus. Kairos Fateweaver and Lorgar arranged things with the Battle of Calth/Ruinstorm/Shadow Crusade precisely to starve Guilliman from information, keep him busy in Ultramar and put him in a position where Plan B looked like the best possible option; all in a concerted effort to keep him and his Ultramarines away from the battle of Terra.
Imperium Secundus' legacy is a source of discussion, for it can be interpreted as Guilliman either being an incredible visionary or a closet traitor wanting power for himself. It was called the "Unremembered Empire" for a reason, officially it didn't actually happen according to the history texts as to do so would portray Guilliman as a traitor against his father.
However, Guilliman's influence on the Imperium after the Horus Heresy is hard to miss: he is widely regarded as the most influential of Primarchs on the Imperium, despite the fact that he did very little to protect it (and possibly even hindered it by accident) during its darkest hour. Sanguinius returned to Terra in time for the final battle, yet neither Guilliman nor the Lion did despite all three starting from the same location, possibly suggesting a conflict between the them, given their actions.
There remains the interpretation that Guilliman's motives were not entirely sincere and that the Emperor, Malcador and the Lion were right: that Guilliman was only interested in building his OWN Empire, irrespective of the Emperor's involvement in it, which is where the problem lies. This is also supported by the the views of his brother Rogal Dorn who had been present at the final battle, returned his dying father to the Golden Throne and actually HEARD his final orders. Considering the Emperor already had knowledge of the war's outcome (read Outcast Dead) he should have prepared his own plan for its aftermath, therefore his orders to Dorn and Jaghatai Khan should have at least involved some sort of planning for the eventuality (unless the Emperor simply revealed what Guilliman was in the process of doing and was going to run with that anyway). Disagreements between them very nearly sparked another civil war about how the Imperium and its military should be run.
Additionally, there remains the fact that 10,000 years later, his own sons are the foremost protectors of the Imperium of Man making up 50-60% of the whole Astartes population and have marginalised the others into either conformity via the Codex or obscurity by denying them descendant chapters. With the Ultramarines Chapter having the singular honour of being the most respected of Space Marine chapters, who set the standard for all other Astartes to aspire to, it's not a difficult set of circumstances to arrange when your father was the one to initially draft the policies that everyone conforms to.
However, immediately after unveiling his Codex Astartes which affirmed that no person, Primarch or otherwise should be able to wield such power of the Legions, Guilliman DID eventually achieve lordship over the Imperium, becoming Lord Commander of the Imperium (which was essentially Warmaster in all but name) and particularly at odds with the stated intention of restricting forces by achieving oversight of ALL armed forces. The position was used up until M32 and subsequent holders were even referred to by the title of "Lord Guilliman". (though the later holders seemed to have significantly reduced influence than Guilliman)
Following Guilliman's resurrection in the 41st Millennium, his first instinct is to secure his 500 worlds and re-establish the stability of Ultramar (and later on he would choose to begin expanding it further) without actually considering the Imperium at large, once again retreading the familiar ground of Imperium Secundus. But in a surprisingly subtle display of character growth, following the infliction of a Nurgle plague that only he can cure by his mere proximity to the afflicted, Guilliman recognises the trap that would tie him to Ultramar. He refuses to commit the same mistake of the past and abandons his Five-Hundred Worlds to the care of Marneus Calgar and sets course for Terra. So at least he now acknowledges that Imperium Secundus was a mistake.
That being said, when he gets to Terra, he resumes the position of Lord Commander of the Imperium following a visit to the Emperor and forcibly replaces High Lords of Terra with candidates of his own choosing, claiming a direct mandate from the Emperor that no-one can verify. Politically expedient and effective? Yes, but his behaviour retains all the hallmarks of a dictator. On the other hand, something similar did happen with Alicia Dominica and the founding Sisters of Battle, and it's not like this would be much of a change for the Imperium anyway. Since then, he has indeed tried to push reforms meant to make the Imperium a better place for everyone, but opposition from the Inquisition and other influential organizations has made such changes slow in coming.
In a stunning show of irony, it was at this time that a gigantic Warp rift began to grow and effectively cut the Imperium in half, so now Guilliman will experience his ill-fated experiment's effects on the Imperium proper firsthand. Odds are he's not going to be happy if he learns the Tyranid invasions are technically his fault as well.
What it means, or heavily implies is that there is possibly an edgier and more relatable side to the Ultramarines beyond simply being Mary Sues or the best thing since sliced bread, and that the supposedly eminent status that the Ultramarines enjoy is because of well calculated political maneuvers. Each of the Primarchs had their strengths which they passed on, in part, to their sons. Thus, if anything Guilliman was said to have been the greatest of statesmen. While the effects of his actions were not all beneficial to the Imperium, one could easily argue that the alternative would be to simply allow the Imperium to fall into complete anarchy in the Emperor's absence.
Shortly after his revival, Guilliman ordered the Ultramarines' Library of Ptolemy to be sealed off with entry forbidden on pain of death. Guilliman claimed the gesture was symbolic of how there would be no place for learning in the dark days they lived in, but what he didn't say was that he wanted to make sure nobody could find evidence of the Imperium Secundus, which he is still deeply ashamed of despite being the only person in a position to know it existed in the first place. It may be possible that he blames himself for the Emperor's mortal wounding as a result of his flawed assumptions.
It is too early to tell in fluff terms how much the position of regent will affect Sanguinius, or him ruling the" Unremembered Empire" will come to affect the later Imperium, as removing the entire Imperium Secundus plotline does not change the opinion of Sanguinius and the Blood Angels one bit. That, and it's not likely to change the fact that Horus will end up killing him either way.
One thing that does remain is the Sanguinor, who was a by-product of this era. As Sanguinius needed a regent to stand for him and deal with petitioners when he was too busy with other important tasks as well as to act as a body-double for the sake of assassination attempts and keep the focus away from the Primarch.
Even at the time, the identity of the herald was kept a closely guarded secret and his name was removed from the records of the legion. By M41, the origins of the Sanguinor are completely forgotten about, even by the majority of the Blood Angels themselves if the ever knew about the practice at all (assuming that the Sanguinary Guard still retain the tradition and that the Sanguinor is not simply Sanguinius resurrected or some sort of daemon-incarnate).
The only thing that can be immediately inferred is that there ought to be a damn good reason why the Blood Angels, after being trapped on the wrong side of the Ruinstorm, end up at Terra WITHOUT the Ultramarines or Dark Angels in tow (it's called shitty writing and bad retcons by GW/Black Library).
Some think the use of the Pharos may have also been the thing that initially got the Tyranids' attention and brought them to the Milky Way, which if true would be highly ironic given the Smurfs' history with the bugs.
Initial evidence for this was circumstantial at best, but fits a pattern. In the First Tyrannic War the bugs made a "bee-line" (pun-intended) directly for Macragge; actually converging on it to a point. When this was thwarted, Hive Fleet Kraken of the Second War was also moving in the "general direction" of Ultramar, but it counted the planet Sotha (where the Pharos was kept) as one of the casualties of war, quite possibly knocking out its effect.
Now compare this to the Third Tyrannic War which attacked the Milky Way from an entirely different axis and seems to have no focus on Ultramar whatsoever and is generally much more dispersed across the galaxy rather than aiming for a particular target.
Again, this a plausible theory that appears to fit known events especially when you look at maps of the hive fleets movements, but without more information on the subject it's quite difficult to tell exactly what caused the Tyranids to appear. Some in-universe sources believe that it might be the Astronomican leading them to the galaxy, like a moth to the flame, but they obviously would never have been aware of the Pharos to account for any possible influence it had.
The epilogue for the book Pharos now pretty much confirms that the Pharos is what brought the great devourer to the Galaxy. That in a brief moment when the Night Lords lay siege to Sotha, the light of the beacon shone brighter than ever before in a last ditch attempt by Warsmith Dantioch to purge the traitors from the planet. The light was noticed from outside of the Galaxy and the hive mind registered it as prey. Oops.