The title of Warmaster, officially designated 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 or early 31st Millennium when it was bestowed upon the most favoured of the Primarchs of the Space Marine Legions, Horus 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 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 - Primarch of the Luna Wolves/Sons of Horus Traitor Legion, Warmaster of the Great Crusade and Arch-Traitor to the Imperium of Man
- 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
- Abaddon the Despoiler - Warmaster of Chaos Undivided and ruling Chaos Lord of the Black Legion of Chaos Space Marines
- Codex: Imperial Guard (5th Edition)
- Codex: Imperial Guard (3rd Edition, 2nd Codex)
- Codex Imperialis
- Warhammer 40,000 Rulebook (5th Edition)
- Sabbat Worlds Crusade (Background Book)