It’s easy to forget that warriors other than Space Marines took part in the Horus Heresy. Despite their prominence, the Legiones Astartes were enormously outnumbered by the human soldiers of the Imperial Army. Uncountable millions of regular, unaugmented troops marched under the Imperial banner – and of those, the Solar Auxilia were the elite.
They are the best trained, best equipped soldiers in the Imperial Army, drilled extensively in both ground and void combat, and renowned as much for their staunch reliability as their combat prowess. If not for the Legiones Astartes, they would be the finest soldiers in the Imperium – an accolade they take great pride in.
Despite being intrinsically linked to the Emperor’s armies, their origins actually stem from the Age of Strife, in the ancient techno-enclaves of Saturn. The Polities of Saturn were formed from the many independent settlements strung across the moons and asteroids of the gas giant, and constant inter-state conflict combined with a technologically advanced culture made them masters of void warfare.
When the Emperor left Terra and took the allegiance of Luna and Mars, Saturn was quick to sign up and supply its own regiments to the Great Crusade. So effective were these troops that soon other regiments directly emulated their weapons, armour, and tactics – since given a once-over by the Mechanicum – and a common warrior heritage began to form between these elite soldiers.
By the time of the Horus Heresy, about a quarter of all human front-line forces were drawn from the Solar Auxilia, thanks largely to their extensive record of battlefield success. Organised into Solar Cohorts numbering some 120,000 soldiers, they were the tip of the spear in fleets that had no Space Marine contingent, although they frequently had the honour of fighting directly alongside the Legiones Astartes.
One such cohort was the infamous Cthonian Headhunters, a rare example of Solar Auxilia drawn from feral or feudal worlds. They held the Sons of Horus in worshipful adulation, and fought alongside their Space Marine lords in some of the most brutal assaults of the Heresy. On the world of Lethe, a sizable group assaulted 52 Imperial Fists Terminators as they broke out from a fortified bunker, and succeeded in bringing down more than half before the sons of Dorn made their escape.
Solar Auxilia cohorts were broken down into smaller sub-cohorts, each roughly equivalent to a Legiones Astartes company, then individual tercios – a fighting unit of approximately 60 troopers, armed with reliable Solar-pattern lasrifles and void-hardened armour that differed little from the original equipment of their Saturn based predecessors. These basic tercios were augmented by a Fire Support group manning tracked Rapier weapon platforms and elite Veletarii tercios, who carried powerful and exotic weapons like volkite chargers.
Their strongest weapon, however, was undoubtedly their discipline, rigorously enforced by pitiless officers throughout the Great Crusade. This allowed them to execute complex manoeuvres in battle that minimised their own losses, such as striking suddenly in force at weakened enemy defences – a vital lifeline once the Horus Heresy pitted them against the vastly superior Legiones Astartes.
Though mostly known for the superlative quality of their infantry, the Solar Auxilia also fielded vast quantities of tanks and artillery, as well as air support seconded from the Aeronautica Imperialis. Their armoured vehicles were designed specifically for their use, featuring void-seals, radiological shielding, and all-round improvements to performance, while specially adapted versions of the Basilisk and Medusa tanks used hard-wired automata to assist their operators.*
Ultimately, the Solar Auxilia proved to be reliable battlefield forces even when fighting the Legiones Astartes, although there was only so much they could do to avoid catastrophic losses against such superior foes. Many cohorts were decimated by the final days of the Siege of Terra, though stories of epic final stands and stubborn defiance against Traitor attack persisted for millennia after.
You’ll be able to command a tercio or five of your own shortly, when Legions Imperialis comes to pre-order. Exactly when we can’t say, but can you hear that pitter-patter of tiny feet in the distance?
* Heavier artillery contingents were provided by other Imperial Army forces, as wise commanders soon realised that rear-line duties were a waste of the Solar Auxilia’s capabilities.