Lord Commander of the Imperium is a singular and unique rank created by the creation of the Senatorum Imperialis - the twelve most powerful individuals in the Imperium's hierarchy - following the disastrous galaxy-wide conflict known as the Horus Heresy. Though history does not record the names of those first High Lords of Terra, tradition of the Ultramarines Space Marine Chapter relates that their Primarch Roboute Guilliman was one of these High Lords. He became the first Lord Commander of the Imperium, the first to carry this singular title and the only man ever to command the entirety of the Imperium's armed forces. This function was formerly administered by the Terran War Council. Under the directives of Primarch Guilliman, he single-handedly reshaped the Imperium, taking the lead role in reforming its administrative and military apparatus following the internment of the Emperor of Mankind within the Golden Throne on Terra. This period of rebuilding and restructuring came to be known as the Reformation.
Guilliman would continue to hold this title, acting as both chairman of the High Lords of Terra and the Imperium's commander-in-chief of its armed forces, until he was fatally wounded by the Daemon Primarch Fulgrim of the Emperor's Children Traitor Legion at the Battle of Thessala in 121.M31, by one of Fulgrim's Chaos-tainted blades. The dying Guilliman was subsequently put into temporal stasis on the verge of death and his body placed upon the throne that lies in the Temple of Correction on the Ultramarines' homeworld of Macragge. The title of Lord Commander of the Imperium continued to be used following Guilliman's internment, and the holder of it continued to serve in both capacities, just as Lord Guilliman did. In honour of the Primarch, the Lord Commander of the Imperium was often simply referred to as 'Lord Guilliman'. This title was no longer used by the modern 41st Millennium, but is known to have been continuously used at least until the mid-32nd Millennium. During this time, known holders of this esteemed title was used by the High Lord Udin Macht Udo and both Chapter Masters Koorland and Maximus Thane. In the aftermath of the War of the Beast and the disastrous event known as The Beheading, the position of Lord Commander of the Imperium was abolished, until the latter years of the 41st Millennium, when the resurrected Roboute Guilliman returned from his millennia-long deathless slumber and returned to Terra during the Terran Crusade, and declared himself as the Lord Commander of the Imperium once more.
Upon his assumption of command, Lord Guilliman forcibly removed several High Lords from office and replaced them with individuals of his own choosing. Guilliman also warned the High Lords of an encroaching darkness, a terrible Warp phenomenon that was even now manifesting itself across the galaxy from end to end. For all the disturbing omens and disastrous losses, Guilliman urged Humanity's leaders not to give up hope. The Emperor of the Imperium was not blind to their plight, and neither was its restored Lord Commander. Under Guilliman's direction, vast armies and armadas were raised in numbers not seen since the Great Crusade that had founded the Imperium ten millennia before. A dark new age called from amidst the fires of endless war, and the Imperium of Man would answer.
- Codex: Ultramarines (2nd Edition) by Rick Priestley, pg. 8
- The Beast Arises (Novel Series) Book Twelve: The Beheading by Guy Haley
- Warhammer 40,000 - The Gathering Storm III: Rise of the Primarch (Campaign Supplement)