"For some, the Great Crusade ended long ago, when the Arch-Traitor Horus brought forth the fires of betrayal and attempted to claim the Imperium for his own. For us, it will never cease until the enemies of mankind are cast down and their chattels laid waste. Only then will we be able to unite the disparate worlds of humanity and usher in a new Pax Imperialis."
The Second Great Crusade is the great period of expansion and never-ending battle to reunite and bring back to the Imperial fold, those worlds that were lost during the galaxy-spanning internecine conflict, known as the Horus Heresy. The Legiones Astartes were left badly depleted, and many of the Loyalist Space Marine Legions en route to Terra, were unable to travel by Warp to bypass the powerful Ruinstorm conjured up by the Traitor Legions. When the newly instilled High Lords of Terra were instilled to oversee the affairs of the mortally wounded Emperor and the ruling of his vast Imperium, they decided to utilise the multitudes of Imperialis Auxilia (Imperial Army) troops as well as the elite formations of the Solar Auxilia to launch the Second Great Crusade, in order to reclaim those worlds lost to the Traitor, to defend the borders of the Imperium from those whom might seek to take advantage of the dire situation, and to bring the sword to those who might threaten Mankind.
Once the Legiones Astartes was brought back to full strength, they too began to take part in the monumental efforts of the Second Great Crusade. During this time, the Space Marine Legions were split to create new legions, to replace those which had fallen to the Ruinous Powers. Since that bygone era, the Imperium has continued to conduct this crusade, which has been ongoing since the end of the Horus Heresy, ten millennia earlier, right up to the present time in the 41st Millennium. Its ultimate goal - to reunite the disparate worlds of humanity and usher in the great "Pax Imperialis Secundus," the Second Imperial Peace across the galaxy.
Following the bitter and bloody fighting of the disastrous civil war, the mortally wounded Emperor was interred within the miraculous workings of the great prosthetic device, known as the Golden Throne. Like the Emperor, the Imperium had been left a dismal shattered thing. As the beauty and grandeur of the Imperial Palace had been burned black in the flames of betrayal, so great swathes of the Emperor's star-spanning realm had suffered a similar fate. Though some bayed for the blood of the Traitors, the Imperium could ill-afford to give chase to the Traitor Legions or even properly prosecute a war of reciprocity.
The resolute guardians of the Imperium, the Legiones Astartes, had been shattered by the betrayal of their former brothers, who had thrown in their lot with the Arch-Traitor, Warmaster Horus. Many of the Space Marine Legions were still beset by the powerful Warp Storm, known as the Ruinstorm, and so were unable to come to Terra's aid in its darkest hour. Rogal Dorn, Primarch of the Imperial Fists and Praetorian of Terra, mourned the loss of his father and lamented the loss of so many innocent lives. But he cautioned the newly instilled rulers of the Imperium, the so-called High Lords of Terra, the Emperor's personal representatives, that the Space Marine Legions needed to be allowed to rebuild and marshal their strength. Their numbers had been sorely depleted during the final epic battle of the conflict, during the Siege of Terra. The three Loyalist Legions that stood alone in defence of Terra needed time to take account of their losses and to lick their wounds. But the resolute Primarch knew there must be some kind of response from Terra. The Traitor Legions needed to be wiped out for their base treachery.
Thus, the High Lords of Terra made their decision and let their judgement be known - though the Legiones Astartes had been gravely wounded, they would live to fight another day. Throwing their lives away needlessly in campaign of vengeance wouldn't benefit anyone. But this did not mean that there wouldn't be any kind of military response. The Imperialis Auxilia, known as the Imperial Army, and the elite regiments of the Solar Auxilia, numbered into the hundreds of millions. Thus, for the first time in over two centuries, the Imperialis Auxilia would be the speartip for the Imperium's military forces, leading the ragged fleets of the Imperium on a Second Great Crusade - to reclaim that which was lost, and to bring the light of reason back to the darkest corners of the galaxy.
But the mortal men and women of the Imperialis Auxilia, though stalwart and brave, were just that - mortal. They were no match for the superior fighting abilities and equipment of the transhuman warriors of the Traitor Legions in open conflict. Though they possessed vast numbers, the Imperialis Auxilia could not hope to best the forces of the Traitor Legions, even though they had been shattered during the Siege of Terra, and fled the wrath of the Imperial forces, they were far from defeated. Knowing that they were outmatched, the Imperialis Auxilia undertook the only course of vengeance left to them - Exterminatus. Instead of taking the fight to the Traitor Legions directly, the Imperial forces would instead take it to each of their individual home worlds. Given the treachery of the Traitor Primarchs and the Imperium's fear that their Chaotic taint would spread, the Imperialis Auxilia enacted the most dire of vengeful acts, bombarding each individual Traitor Legion home world with cyclonic torpedoes. The deadly cargo carried by these munitions was the Life-eater Virus, one of the most terrible Exterminatus-class weapons in the Imperial arsenal, and a weapon whose use only could be authorised by a Primarch or the High Lords of Terra themselves. Billions died as each Legion home world became a lifeless husk.
Though the Imperial forces were initially successful with the destruction of the various Traitor Legion home worlds, they soon found themselves woefully unprepared to face the multitude of threats that threatened Mankind from all corners of the galaxy. The Imperium fought not only the traitors but also various xenos that took advantage of the turmoil caused by the Horus Heresy to plunder human worlds. Systems once thought unassailable to the xenos threat were put under siege. Entire sectors had to be cordoned off until sufficient forces could be concentrated to overwhelm the myriad threats, in what would certainly be gruelling and bloody campaigns of compliance.
As Mankind teetered on the very brink of annihilation, the greatest heroes of Mankind came to the fore to unite in defence of His realm. Rogal Dorn rallied the surviving loyalist warriors and, together with his Imperial Fists Legion, stood as bulwark against the tide of enemies that sought to deliver the deathblow to the Emperor's dominion. Dorn and his stalwart Space Marines held the scattered defenders of Humanity together through the nightmare that followed. Legends tell that Dorn was everywhere in these times, rallying defenders on one world and leading them to victory before moving on to another where his awe-inspiring skills might best serve humanity. Though invaders circled like vultures, the multitudes of the Imperialis Auxilia and the Solar Auxilia, augmented by the Space Marines of the Imperial Fists, and later, the White Scars and Blood Angels Legions, stood firm.
Darkness looked sure to overwhelm them at every turn, but not one amongst them took a backwards step. With the Segmentum Solar and its galactic borders finally secured, the Legiones Astartes could finally marshal their strength for the monumental undertaking set before them. It would take many more years for the remaining loyalist Space Marine Legions to reach Terra with the slow dissipation of the Ruinstorm that spanned much of the known galaxy. Once reunited with their fellow legions on sacred Terra, it would take several centuries until the Space Marine Legions were finally brought back to full strength. Once this was accomplished, the Second Great Crusade could finally begin in earnest with the newly rearmed and re-equipped Space Marine Legions at the forefront of the Imperium's conquering armies.
After the proclamation of the Second Great Crusade, the High Lords of Terra had despatched the Imperialis Auxilia and Solar Auxilia forces to reclaim that which was lost during the Heresy, and to bring the light of the Emperor to the darkest corners of the galaxy. But without the Legiones Astartes to spearhead the reclamation of the Imperium, who were all but shattered during the bloody galactic civil war, things were not going well for the Imperial crusade. While the forces of the Imperial military continued to fight a desperate campaign against the Traitor forces and those who colluded with them, a large enclave was called upon Terra. Though it took many months for all the Loyalist Legions to make their way to Terra due to the vagaries of the warp and the unpredictability of the Ruinstorm, eventually they all arrived on the Imperium's capital world. This conclave was held deep within the bowels of the once majestic Imperial Palace, which still lay cracked and broken in many places from the Traitor orbital bombardment and the fighting that had raged within its sacred walls. This gathering was comprised of the mightiest Space Marine Legions; Dark Angels, Imperial Fists, Blood Angels, Ultramarines, White Scars, Iron Hands, Salamanders, Raven Guard and the Space Wolves. They had answered Rogal Dorn's summons to discuss the future of the Legiones Astartes.
Before the assembled throng, seven of the remaining nine Loyalist Primarchs were seated in simple leather seats, specifically made for their massive frames, on a raised dais in the center of an ancient auditorium. Among those present were Lion El'Jonson of the Dark Angels, Jaghatai Khan of the White Scars, Ferrus Manus of the Iron Hands, Vulkan of the Salamanders, Corax of the Raven Guard, and Leman Russ of the Space Wolves. The assembly was presided over by the Praetor of Terra himself, Rogal Dorn of the Imperial Fists. Two of the seats were unoccupied, left in honour of the two Brother Primarchs who had been tragically killed during the recent events of the Heresy - Roboute Guilliman of the Ultramarines, and more recently, Sanguinius of the Blood Angels.
The Ultramarines Primarch Roboute Guilliman, who had been held as a paragon among the Emperor's sons, had fallen to the bolters of treacherous Alpha Legion assassins. They had been disguised in captured Ultramarines' war-plate that appeared battle-worn from months of toil and warfare upon the radiated surface of Calth fighting against the treacherous Word Bearers. They had arrived at Macragge Civitas, the Ultramarines' formidable fortress-monastery on the capital world of Ultramar, and were given personal audience with the Primarch. Once inside the Primarch's personal chambers, they had mag-locked the door - locking Guilliman in with the ten would-be killers. When the Invictarii guard finally managed to cut their way through the main door, they found Guilliman lying prostrate upon the marble floor, surrounded by a massive pool of his own blood. Though he had put up a valiant fight, killing nine of his attackers with only his bare hands, the sole remaining assassin managed to to slay Guilliman with a single bolter round through one of the Primarch's eyes and into his brain. When the enraged Ultramarines entered the room, they found the lone assassin unarmed, and bleeding profusely as he stood over the fallen Primarch. Before the senior Invictarus gave the command to open-fire upon their Primarch's killer, he inquired, "Who? Who are you?" The killer replied, "I am Alpharius." In response, the Invictarii killed the assassin in a hail of mass-reactive rounds.
The news of Guilliman's death spread like wildfire across the Realms of Ultramar - incredulously, one of the Emperor's nigh immortal sons had been slain. They were indeed mortal after all. Guilliman's death, unfortunately, would not be the last. One other Primarch would also tragically meet his end during the final epic battle of the Heresy, during the Siege of Terra. The angelic Blood Angels Primarch Sanguinius, despite receiving prescient visions of his own death, willingly went with the Emperor to assault Warmaster Horus' flagship, the Vengeful Spirit, in order to confront the Arch-Traitor Horus and end the bloody conflict. But during their teleportation from the Imperial Palace to the Warmaster's vessel in orbit above Terra, the Arch-Traitor had used malefic means to separate the Loyalist forces. Separated from his comrades, Sanguinius was brought through artifice before the treacherous Horus. The Warmaster offered a place at his right side to his Brother Primarch to rule over the Imperium, if only he would renounce the Emperor and kneel in service to the Warmaster. But the Red Angel refused, and fought back against his erstwhile brother, despite knowing how such a one-sided confrontation would inevitably play out. In the end, Sanguinius was slain by Horus. When the Emperor finally managed to fight his way into the Warmaster's chambers and saw his fallen son lying upon the floor, broken and bleeding, the Emperor became enraged. Putting aside any thoughts of compassion for his former favoured son, the Emperor slew the corrupted Arch-Traitor with his psychic might, though the confrontation cost him dearly, as he was wounded unto death.
Gathered before the lauded assembly, Rogal Dorn presented his proposal. The Legiones Astartes should not only be kept intact, but they should seek to rebuild their numbers, and eventually expand them even further. To help guide these future Legions, they would fall back on the wise tenets of the Principia Belicosa, the ancient tome of military organisation, strategy and tactics of ancient Roma that was first utilised by the nascent Legiones Astartes in regards to disciplined structure and organisation. This sage wisdom was taken from the ancient and proven Terran patterns of strategy, hierarchy and functions as laid down in this revered text, as well as Krom's fragmentary New Model, that had survived in the hands of the tyrants of Old Earth down the blood-stained generations of the Age of Strife. To these venerable treatises the Emperor and his commanders had added their own genius and created a sturdy but adaptable strategic framework that spoke to the fundamental strengths and superhuman abilities of the Legionaries themselves. Rogal Dorn sought to add his own distilled wisdom to the Principa Belicosa, but knew in his hearts, that he was not the tactically brilliant strategist and organiser that his brother Roboute Guilliman was. During his research into other ancient tomes and tactical doctrine compiled by Imperial war savants, he came across a copy of a beautifully written but unfinished work - Guilliman's proposed treatise, entitled the Codex Astartes. Before his death, the Ultramarines' Primarch had sent Dorn a copy of his proposed treatise for his brother's review and approval. But the Praetor of Terra had been preoccupied with overseeing the defence of Terra for the Traitor Legions' inevitable assault of the Imperium's capital world.
Dorn carefully scrutinised his brother's work and was left in awe by his brother's tactical brilliance and keen foresight. This was surely Guilliman's finest work - his magnum opus. Within its vellum pages, the Ultramarines Primarch had written upon multitudinous topics concerning various tactics, deployment strategies and logistics. Typical of Guilliman, he had foreseen almost every possible tactical scenario, as well as proposed theoreticals and practicals. Dorn also found Guilliman's ideas for the reorganisation of not only the various Adepts of the Imperium, but also it's military forces as well. Guilliman had also postulated that in order to purge certain perceived weaknesses within the Legiones Astartes, he proposed that in the future, Space Marines should forevermore be created and trained slowly. The genetic banks used to create the Astartes' implants should also be carefully monitored and scrutinised for any defects and subjected to the most stringent of purity tests. Amongst Guilliman's more radical ideas, Dorn also discovered the contentious topic of breaking up the existing Space Marine Legions, and reorganising them into smaller existing organisations known as Chapters. This would ensure that no one individual would ever hold power over an entire Space Marine Legion ever again. Though Dorn implemented many of Guilliman's ideas and strategies into the Principia Belicosa, Guilliman's ultimate vision for the Legiones Astartes, unfortunately, would never come to pass.
Dorn and his fellow Primarchs debated for hours amongst one another about the merits and flaws of Dorn's proposed revisions of the Principia Belicosa. Many of his fellow Primarchs decried Dorn's proposal, as some of the Legions, such as the White Scars and Space Wolves, had abandoned the strictures of the Principa Belicosa and could no longer even notionally apply them, as they had developed their own distinct character and preferences in the organisation, tactical strategies and deployments of their individual Legions. But Dorn assured them, the Principia Belicosa would only be utilised as a basis from which to build upon, as he had no desire to force his brothers to change their Legions' individuality. He doubted he could, even if he so desired. With the final approval of the majority of the assembled Primarchs, Rogal Dorn presented his proposal for the future of the Legiones Astartes to the High Lords of Terra, the newly-instilled representatives of the Emperor, who ruled by proxy on His behalf. The High Lords acquiesced to Dorn's proposal, and adopted his additions to the Principia Belicosa.
The Horus Heresy had revealed weaknesses in the gene-seed of several Space Marine Legions. These defects had been exacerbated by the accelerated gene-seed cultivation techniques needed to keep the huge Space Marine Legions up to strength. Dorn believed that the Chaos powers were able to exploit the resultant physical and mental corruption to turn Horus' troops against the Emperor. The High Lords of Terra ordered the Imperium's most brilliant gene-wrights and fleshcrafters to expunge these genetic weaknesses. Working in conjunction with the Selenar gene-cults of Luna and the Magos Biologis of Mars, they focused on perfecting and removing the existing deficiencies in the identified flawed gene-seed, with the ultimate goal of producing new and improved Primarch-like Space Marines as part of what was called 'Project Homo Sapiens Novus'. Unfortunately, the genetors proved far less-skilled in the genetic sciences than the Emperor of Mankind, but they did succeed in achieving a way of improving upon the existing gene-seed to a level that was above that of an ordinary Astartes, but less than a Primarch, making a subject almost as comparable as the Emperor's own elite Legio Custodes.
By utilising a small portion of the existing base stock of pure Primarch DNA, they would be able to created an entirely new strand of gene-seed. Unfortunately they would only be able to make enough for perhaps two dozen individual Astartes, at most, as they supply of the Primarch DNA was finite. This new-and-improved purestrain gene-seed would only be given to those veteran Astartes who had shown absolute loyalty to the Emperor and who had gone above-and-beyond during the tragic events of the Heresy. These newly enhanced Astartes were to be earmarked for command of newly created Legions, whose gene-seed themselves, would be derived from existing gene-seed, after having many of their genetic flaws and deficiencies removed. Thus, the Second Founding would occur at the beginning of the 32nd Millennium, where two of these newly created Legions would be stood up, to replace the ones that had fallen to Chaos during the Horus Heresy.
As the dawning of the 32nd Millennium rapidly approached, the High Lords of Terra declared that a Second Founding would occur. This new and improved gene-seed was given to a new generation of Space Marine Legions created from existing Loyalist Space Marine Legions to replace those Legions who had fallen to Chaos, and followed Warmaster Horus into damnation and eventual ruination during the Horus Heresy. However, as the centuries dragged on, it became apparent that even the most capable genetors of the Imperium were no match for the Emperor of Mankind's genius, and the flaws inherent in the First Founding Legions persisted through to their successors, albeit in a diminished capacity. The Second Founding of the Legiones Astartes occurred in 091.M32. Legions of the Second Founding became known for their closeness with their progenitors, as many of their first ranks were drawn directly from the First Founding Legions. The Second Founding Legion are often noted for their long, honoured legacies and stalwart dedication to the Second Great Crusade. While other Legions will prioritize domestic defence or internal conflicts within the Imperium over border conflicts, the Second Founding Legions are often more concerned with conquest and recapturing the lost glories of the First Great Crusade. Many bear with them icons of the Unification Wars as rallying symbols, and hold stridently to the founding principles of the Imperium of Man.