|Head of state:||Sanguinius as Imperator Regis|
|Governing body:||None, ruled by triumvirate of Sanguinius, Lion El'Jonson, and Roboute Guilliman|
|State Religion:||None, Imperial Truth enforced|
Imperium Secundus was a Human empire on the Eastern Fringe of the Galaxy formed in 009.M31 during the Horus Heresy. At its peak, it compromised roughly five hundred worlds and was centered around the realm of Ultramar.
As a result of massive Ruinstorm created by Erebus in the Shadow Crusade the Astronomican had been thrown into a state of disarray, rendering navigation and communication impossible and effectively dividing the Imperium in half. Seeing the dire state of humanity, Primarch Roboute Guilliman saw fit to enact a contingency plan and create a new Imperium should the power of Terra fall. Guilliman had little knowledge of what was transpiring outside of Ultramar, and decided that rebuilding the Imperium was a necessity even if he was not sure if the Imperium was not yet destroyed.
Thanks to the loyalist Warsmith Barabas Dantioch activating the Pharos, warriors of the Emperor and refugees fleeing the Shadow Crusade were able to find their way through the Ruinstorm and make it to Macragge. Soon, Guilliman was joined by his brother Lion El'Jonson and the Dark Angels. The comatose body of Vulkan was eventually brought to Imperium Secundus as well, joined by scattered forces of the Salamanders under Zytos, Raven Guard under Verano Ebb, Iron Hands under Eeron Kleve, White Scars under Timur Gantulga, Imperial Fists under Alexis Polux, and Space Wolves under Faffnr Bludbroder. Sanguinius and the Blood Angels, fresh from the Battle of Signus Prime, were among the last to arrive.
Not wanting to fall into the same trap of self-aggrandizement as Horus, Guilliman refused to crown himself Emperor. He proposed to Lion El'Jonson that he rule Imperium Secundus. However ultimately, the duty would fall to a reluctant Sanguinius, Primarch of the Blood Angels, who was named Imperator Regis. Guilliman became Lord Warden while Lion El'Jonson was named Lord Protector of Imperium Secundus, a title similar to Warmaster. A major test for Imperium Secundus came during the Battle of Sotha, where Imperial forces attempted to defend the Pharos. Fortunately for Guilliman, his forces were victorious. After the battle, Guilliman and Lion El'Jonson came to severe disagreement over what to do about Night Lords Primarch Konrad Curze, who was still loose upon the planet. Curze may have been behind a string of suicide bombings and rebellions that targeted Guilliman's authority on Macragge, resulting in Lion El'Jonson using the Dark Angels to put Macragge under martial law and deploying the Dreadwing to devastate rebellious areas of the planet. However Curze was ultimately captured by The Lion in the Illyrium region and forced to stand trial.
Meanwhile on Terra, Malcador the Sigilite suspected that Guilliman may try to build his own empire, and was cautiously nervous that the Primarch may be attempting to usurp the Emperor's power. Malcador even hoped that the Dark Angels would stop Guilliman.
By 014.M31, it became clear that the Emperor and Terra endured, thanks in large part due to the visions of Sanguinius and the captive Curze. The Triumvirate of Imperium Secundus decided to abandon their project, which many now considered redundant and heretical. Imperium Secundus was abolished and its territories were once again declared part of the Realm of Ultramar under the Regency of Valentus Dolor. Guilliman, Sanguinius, and Lion El'Jonson led the bulk of their respective legions through the Ruinstorm in hopes of reaching Terra and aiding the Emperor in his final struggle, though ultimate only Sanguinius and the Blood Angels would succeed in this.
- 1: The Unremembered Empire
- 2: Vengeful Spirit (Novel)
- 3: Pharos (Novel)
- 4: Angels of Caliban (Novel)
- 5: Ruinstorm (Novel)