- "You are like a son and together we have all but conquered the galaxy. Now the time has come for me to retire to Terra. My work as a soldier is done and now passes to you for I have great tasks to perform in my earthly sanctum. I name you Warmaster and from this day forth all of my armies and generals shall take orders from you as if the words came from mine own mouth. But words of caution I have for you for your brother Primarchs are strong of will, of thought and of action. Do not seek to change them, but use their particular strengths well. You have much work to do for there are still many worlds to liberate, many peoples to rescue. My trust is with you. Hail Horus! Hail the Warmaster!"
- —The Emperor of Mankind, the Triumph of Ullanor
The title of Warmaster, officially designated in the modern era by the Administratum as Lord Solar, is one of the highest military ranks and titles bestowed upon an individual by the Imperium of Man. The title originated in the Golden Age of Imperial history in the final days of the 30th Millennium when it was bestowed upon the most favoured of the Primarchs of the Space Marine Legions, Horus Lupercal of the Luna Wolves, by his father the Emperor of Mankind. The title was created in recognition of Horus' promotion by the Emperor to take His place as the supreme commander of all Imperial military forces during the latter half of the Great Crusade, after the Emperor had retired to Terra to begin work on his secret project to open the Eldar Webway up to human use. The title was officially granted to Horus during the great Imperial triumph held after the successful conclusion of the campaign known as the Ullanor Crusade against the Orks. Eventually, the Luna Wolves were renamed the Sons of Horus by Horus to better reflect the prestige of the Warmaster's title following his corruption by the Dark Gods. At present, the title of Warmaster is one of the most powerful ranks that can be granted to an officer of the Imperial armed forces, including officers of the Imperial Guard, the Imperial Navy or the Chapter Master of one of the Adeptus Astartes Chapters who is commanding a much larger Imperial force. The rank is officially second in the Imperial military hierarchy only to that of the Lord Commander Militant of the Imperial Guard, who serves on the Senatorum Imperialis as one of the High Lords of Terra and is the commander-in-chief of the Imperial Guard.
- "The opportunity that we are offered is far too great to ignore. A brief foray, using the assets of the Calixis Sector, could spread the glory of the God-Emperor to millions who crave his light and return untold wealth and glory to Scintilla."
The title of Warmaster is bestowed by the High Lords of Terra in the present Imperium when an Imperial Crusade is being planned and resources from multiple Imperial sectors are needed. The supreme overall command designated by the rank of Warmaster is necessary as inter-and intra-service strife and bureaucratic rivalries might make assembling the materiel required and co-operating in the field difficult for officers of the different services without a clear grant of supreme authority. The rank is not available unless granted by the High Lords of Terra and it is the normal formulation to say that such an individual has been given his or her command by the Emperor himself. There is rarely more than one Warmaster operating within the Imperium at any one time, due to the extreme powers given to the holder of the title.
Some in the Imperium consider the title of Warmaster to be cursed or a sign of ill-omen because of the ties of the rank to Horus and the Horus Heresy, the terrible galactic civil war that tore the Imperium in two. This is one reason why the title is rarely granted, and also the reason the title is officially proclaimed as "Lord Solar" when issued. It is also the reason why the Forces of Chaos now grant that title to the Chaos Lord who is capable, like Horus, of uniting all the many different Chaotic factions under the banner of Chaos Undivided in pursuit of the single goal of destroying the Imperium of Man in the Long War. At present, the greatest Chaotic Warmaster is Abaddon the Despoiler, though the title is often claimed by any Chaos Lord strong enough to unite a disparate group of the servants of the Dark Gods under his command.
- Horus Lupercal - Primarch of the Luna Wolves/Sons of Horus Traitor Legion, Warmaster of the Great Crusade and Arch-Traitor to the Imperium of Man.
- Abaddon the Despoiler - Warmaster of Chaos Undivided and ruling Chaos Lord of the Black Legion of Chaos Space Marines
- Macharius - Lord Commander Solar and leader of the Macharian Crusade.
- Solon - Successor to Macharius as Lord Commander Solar and Warmaster of Imperial forces during the Macharian Heresy
- Slaydo - The first Warmaster of the Sabbat Worlds Crusade.
- Macaroth - The second Warmaster of the Sabbat Worlds Crusade and successor to Warmaster Slaydo.
- Ingenus - In 845.M41, the Dimmamak War escalates after Dark Eldar pirates capture Warmaster Ingenus. In response, a further forty thousand regiments are raised and all high-ranking Imperial Guard officers are assigned personal bodyguards.
- Tiber Achilus - Tiber Achilus was the catalyst of nearly every move the Imperium's forces made during the Achilus Crusade into the Jericho Reach, even influencing the decisions and actions of those only nominally under his control such as the Inquisition, the Ecclesiarchy, the various Space Marine Chapters, and the Deathwatch.
- Codex: Imperial Guard (5th Edition), pp. 7, 13, 22
- Codex: Imperial Guard (3rd Edition, 2nd Codex)
- Codex Imperialis
- Deathwatch: The Achilus Assault (RPG), pg. 21
- Warhammer 40,000 Rulebook (5th Edition)
- Sabbat Worlds Crusade (Background Book)
- Horus Rising (Novel) by Dan Abnett
- 13th Legion (Novel) by Gav Thope