The Lord Commander of the Imperium is a singular and unique rank held by the individual who serves as the first among equals in the Imperium of Man's ruling oligarchy, ruling Humanity in the name of the Emperor of Mankind. It has been held only a few times in the Imperium's history, in the first case in the centuries after the end of the Horus Heresy in the early 31st Millennium and most recently again after the Terran Crusade of 999.M41. In both the first and last instance, the position was filled by the same man -- the Primarch Roboute Guilliman.
The position was created by the Senatorum Imperialis -- the twelve most powerful individuals in the Imperium's hierarchy -- following the disastrous galaxy-wide interstellar civil war that was the Horus Heresy. Though history does not record the names of those first High Lords of Terra, the tradition of the Ultramarines Space Marine Chapter relates that their Primarch Roboute Guilliman was one of those 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. Before the Heresy, this function had been administered by the Terran War Council. Guilliman's directives single-handedly reshaped the Imperium, restructuring its administrative and military apparatus following the internment of the Emperor of Mankind within the Golden Throne on Terra. This period of rebuilding came to be known by later Imperial savants as the Reformation.
Guilliman would continue to hold this title, acting as both Chairman of the High Lords of Terra and the Imperium's military commander-in-chief, until he was fatally wounded by the Chaos-tainted blade of the Daemon Primarch Fulgrim of the Emperor's Children Traitor Legion at the Battle of Thessala in 121.M31. 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 for a time following Guilliman's internment, and the later Lord Commanders continued to serve as both first among equals on the Senatorum Imperialis and as the commander of the Imperium's military. In honour of the Primarch, the Lord Commander of the Imperium was often simply referred to as "Lord Guilliman." This honorific was no longer in use by the late 41st Millennium, but is known to have been in continuous use at least until the mid-32nd Millennium. During this time, the known men known to have served as Lord Commander of the Imperium included the High Lord Udin Macht Udo and the Adeptus Astartes 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 last years of the 41st Millennium, when the resurrected Roboute Guilliman returned from his millennia-long dreamless slumber and returned to Terra during the Terran Crusade. Following his meeting with his father the Emperor for the first time in ten thousand standard years, Guilliman informed the High Lords of Terra that he was once more taking up the mantle of the Lord Commander of the Imperium.
Upon his resumption of the rank, Lord Guilliman forcibly removed several High Lords from office and replaced them with individuals of his own choosing. The reborn Primarch also warned the High Lords of an encroaching darkness, a terrible Warp phenomenon called the Great Rift 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 - Book Twelve - The Beheading (Novel) by Guy Haley
- The Gathering Storm - Part Three - Rise of the Primarch (7th Edition)