Imperium Secundus

• It's Guilliman, What do you think he's doing? He's building an Empire! - Malcador the Sigillite

Formation

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 defence.

He also insisted that he would not be it's 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. Thus to prove his point he would 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 / the Ultramarines sincerity:

Misrepresentation

Warsmith 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.

This is not entirely unforgivable though, as the 500 Worlds were cut-off from Terra, so Dantioch might never have been able to get back. Plus as a Loyalist, Dantioch and his men would have been mistrusted, therefore being absorbed into Ultramarines would have been as wise a choice as any.

Mustering Forces to Himself

What Guilliman found on Sotha was a pre-Imperial device that acted similarly to the Astronomican: Called 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. 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.

In addition, the Pharos caused the Imperial Fists Retribution Fleet, notable for being the largest fleet available to the Imperium, fresh from the Battle of Phall to arrive at Macragge. ...AND the entire Blood Angels Legion (except for about eleven men left on Baal, the "Lost Sons") from the Battle of Signus turned up on Guilliman's doorstep too.

Both the Blood Angels and Imperial Fists were actually looking for Terra, so it could definitely be said that all these arrivals would starve Terra of valuable resources and actually hinder the war effort. In fact this was explicitly stated to Lorgar by the Fateweaver prior to the battle at Calth, that if Guilliman died, then Horus would have no chance at the Siege of Terra.

In fact, if Guilliman had just shone the Pharos at Terra, the war could possibly gone the other way as Terra received the intended reinforcements.

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 argued that it was to the Imperium's benefit and to be of service to his father.

Here, the Lion was very clear on the issue with his brother Guilliman, seeing his brother's new Imperium as an threat equally as grave as Horus to the kingdom that his Father had built.

When the Lion arrived on Macragge, the issue of who should be the ruler did come up, but neither the Lion and Roboute came to blows with one another over the matter and unanimously elected that Sanguinius take the position of Regent when his fleet arrived. So at least both brothers could be described as sincere on this one.

Legacy

It was called the "Unremembered Empire" for a reason, it didn't actually happen according to the history tests as to do so would portray Guilliman as a traitor against his father.

However, Guilliman's influence on the Imperium after the Horus Heresy are hard to miss, since 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) during its darkest hour, Sanguinius returned to Terra in time for the final battle, yet Guilliman nor the Lion did despite all three starting from the same location, possibly suggesting a conflict between the two, given their actions.

Guilliman would eventually achieve the position of lordship over the Imperium, becoming the first Lord of Terra and immediately abdicating after the production of his Codex Astartes which affirmed that no person, Primarch or otherwise should be able to wield such power.

However, there remains the suggestion that he was still not entirely sincere and that the Emperor, Malcador and the Lion were right.

Ten-Thousand years later, his own sons are the foremost protectors of the Imperium of Man, making up 50-60% of the whole Astartes population, with the Ultramarines Chapter having the singular honour of being the most respected of Space Marine chapters, which set the standard for all other Astartes to aspire to. Not a difficult set of circumstances to arrange when your father was the one to initially draft the policies that followed.