Originally, this post was held by an ordinary Space Marine Legionary in the absence of their missing Primarchs, who had been mysteriously spirited away from the Emperor's gene-labs on Terra by the Ruinous Powers in the days before the start of the Unification Wars.
In the absence of the Legion's Primarch, command of a Space Marine Legion was given to a select few -- Veteran Astartes warriors who were chosen for their tactical acumen, steadfast dedication and loyalty to the Emperor of Mankind and His Great Crusade as well as their unmatched personal combat prowess among their battle-brothers.
Prior to the rediscovery of the Primarchs, the Legion masters represented the highest echelon attainable within a Space Marine Legion.
As the Great Crusade progressed and the missing Primarchs were eventually found, the Astartes gene-sires would then assume command of their respective Legions, replacing the Legion master as overall commander of the force.
List of Notable Legion Masters
The following is a list of all known Legion Masters that commanded one of the Space Marine Legions:
- Thrallas - Thrallas was the Legion Master of the III Legion, later the Emperor's Children, before their reunification with their Primarch Fulgrim on the world of Chemos.
- Enoch Rathvin - The Terran-born Enoch Rathvin was the Commander of the VI Legion, later the Space Wolves, during the early years of the Great Crusade. It was during the Compliance of "1-122", a world also known as Delsvaan, where the VI Legion quickly garnered a reputation under his leadership as a ferocious, unforgiving, but steadfastly loyal formation. Following their reunification with their Primarch Leman Russ upon Fenris, the increasingly unstable Enoch Rathvin met his fate during the xenocidal campaign against the Orks, within the region known as the Wheel of Fire, when he was crushed in the hydraulic claws of an Ork hell crawler while leading a suicidal charge on the world of Xyat.
- Mathias - Mathias was the Terran-born Legion Master of the VII Legion, the Imperial Fists, before their reunification with their Primarch Rogal Dorn on the world of Inwit. Thanking him for his service, Dorn named the former Legion Master as High Castellan of the Inwit Cluster. Such an honour was also an onerous duty, for Mathias' next command was to raise thirty regiments of new Imperial Fists from the Inwit System.
- Ishidur Ossuros - Ishidur Ossuros was the first and only Legion Master of the IX Legion of Space Marines before it was reunited with its Primarch Sanguinius and renamed the Blood Angels. In truth, the real Ossuros died early on in the Great Crusade, but his identity and memories were taken by his successors though the abilities granted by their Legion's overactive Omophagea gene-seed implant. They allowed his legend to live on through them until their own deaths. The last incarnation of Ishidur Ossuros fell during the first battle of the newly-named Blood Angels with their Primarch, when he gave his life to save Sanguinius from being overrun and slain. Ossuros' body could not be found, so his persona could not be passed on to a new Astartes through the consumption of his flesh.
- Amadeus DuCaine - Lord Commander Amadeus DuCaine was the commander of the X Legion, the Storm Walkers, during the latter years of the Unification Wars and the early years of the Great Crusade in the late 30th Millennium before the rediscovery of the X Legion's Primarch, Ferrus Manus and its renaming as the Iron Hands. He died fighting in orbit above Istvaan V during the Drop Site Massacre against vessels of the Traitor fleet.
- Ibram Ghreer - Ibram Ghreer, a Terran-borne Astartes, was a respected general and the Legion Master of the War Hounds who commanded the XII Legion for nearly three standard decades until he disappeared without explanation from his taciturn Legion. In truth, he was killed by the Primarch Angron within the bowels of the flagship Conqueror so that the Primarch could assert his rightful command over the XII Legion whose Astartes had been created from his own genetic material. Ghreer was considered to be an axeman to stand with the best of the XII Legion's warriors. He had proved to be a poor strategic planner, yet had managed to transform bluntness into a virtue for the XII Legion alongside brutality.
- Lhorke - Lhorke, also known as "the First," was the Legion Master of the XII Legion, the War Hounds, prior to the rediscovery of the Primarch Angron and its renaming as the World Eaters. However, after being grievously wounded on the world of Jeracau, he was interred within the shell of a Contemptor Dreadnought and was forced to relinquish his command of the Legion to another. Given his position of respect and new status as a Dreadnought, he was never implanted with the Butcher's Nails. After spending the Istvaan III Atrocity in stasis, he was only reawakened during the Shadow Crusade, where he met his end at the hands of Angron on Nuceria.
- Gren Vosotho - Gren Vosotho was a part of the first induction of Terran initiates for the newly-formed XIII Legion, the War-Born, later the Ultramarines, during the latter years of the Terran Unification Wars. Showing great promise as a leader, Vosotho was selected for high command, eventually rising to the esteemed position as the first Lord Commander of the XIII Legion, leading them for over three solar decades. Vosotho met his fate at the hands of unknown xenos designated the "Psybrids" when the XIII Legion were sent to put down the secessionist movement within the Osiris Cluster. Vosotho committed his life to the command of the rearguard on the surface, taking penance for his error, and his last act was to transfer Legion command to the most senior surviving officer present in orbit, Marius Gage.
- Halik-gar - Halik-gar was the High Herald of the XVII Legion, the Imperial Heralds, and its Legion master and commander prior to the rediscovery of the Primarch Lorgar and his renaming of the Legion as the Word Bearers.
- Cassian Vaughn - The first Lord Commander of the XVIII Legion, the Dragon Warriors, later the Salamanders, from the time it was founded on Terra during the Unification Wars until the discovery of the Primarch Vulkan on the world of Nocturne during the Great Crusade. Cassian was mortally wounded in battle against the Orks, but such was the esteem in which Vulkan held this warrior that he undertook to fashion for him a unique Dreadnought sarcophagus known as the Dracos Revenant, or the Iron Dragon. During the Drop Site Massacre on Istvaan V, he managed to survive where most of his Legion had perished. Cassian Vaughn's tale becomes intermingled with that of the famed Ebon Drake, alongside which he would return to the battlefields of the 31st Millenium, most notoriously during the Third Siege of Mezoa. His final fate remains unknown.
- Arkhas Fal - Shade Lord Arkhas Fal was the Terran-born Master of the XIX Legion before they were united with their long-lost Primarch Corvus Corax on the world of Deliverance. Once given command of his Legion, Corax began to impose the style of war he had perfected upon Lycaeus over that which had defined the XIX Legion of old, particularly by purging the more cold-blooded ways of the Terran Xeric tribes which had once defined the Legion culture. The Legion had so often served in oppression, repression and occupation forces that Corax saw in some of the Terran members of his Legion something akin to the slavers of Lycaeus. Several of the Legion's highest ranking officers were displaced or reassigned to non-command roles, including Shade Lord Arkhas Fal. A small number of mostly Terrans of the Legion of old had been assigned to posts beyond the fringes of the Imperium, some in independent nomad fleets, one of which was commanded by Arkhas Fal. What became of the Shade Lord, his fleet and the other elements may never be known, and the fact that Corax appears to have made no effort to recall them suggests they were considered a body apart from the bulk of the Legion.
- The Horus Heresy - Book Two: Massacre (Forge World Series) by Alan Bligh, pp. 65, 117-118, 138
- The Horus Heresy - Book Three: Extermination (Forge World Series) by Alan Bligh, pg. 60
- The Horus Heresy - Book Five: Tempest (Forge World Series) by Alan Bligh, pp. 73-75
- The Horus Heresy - Book Seven: Inferno (Forge World Series) by Alan Bligh, pp. 70-75, 80
- The Horus Heresy - Book Eight: Malevolence (Forge World Series) by Neil Wylie and Anuj Malhotra, pg. 123
- The Flight of the Eisenstein (Novel) by James Swallow, "Chapter Six"
- Betrayer (Novel) by Aaron Dembski-Bowden
- Wolfsbane (Novel) by Guy Haley, Prologue
- After Desh'ea (Shorty Story) by Matthrew Farrer
- Chirurgeon (Short Story) by Nick Kyme
- Meduson (Anthology) by multiple authors, "Meduson," (Short Story) by Dan Abnett
- Corax (Anthology) by Gav Thorpe, "The Weregeld," (Short Story) by Gav Thorpe