The desperate rebels had opened hidden vaults deep beneath the surface of the planet and set loose a biogenic phage that had reshaped the broken people of their world into twisted, blood-hungry ghouls whose minds were burned clean of all thought except the need to hunt and kill. These monstrous mutant creations then fell upon the unsuspecting warriors of the XIII th Legion with a ferocity that gave pause to even the warriors of the Legiones Astartes. The three primarchs were instrumental in the foundation of the " Imperium Secundus" as a means of continuing the fight against the Traitors and securing the Emperor's great work. Guilliman proclaimed Sanguinius as the rightful heir to the Emperor and declared him the new ruler of Imperium Secundus. They feature in the novel Angels of Darkness, the The Legacy of Caliban trilogy, and the Space Marine Battles novel The Purging of Kadillus, all by Gavin Thorpe. They also feature in the Horus Heresy novels Descent of Angels and Fallen Angels, Azrael, and The Lion: Son of the Forest. How is this all explained to the ~80% of DA and DA successors who have no idea of the Fallen's existence? This bitterly contested campaign, known as the Thramas Crusade, dragged on for nearly three standard years. In an attempt to sway his brother Lion El'Jonson to Horus' cause, the Night Haunter left a deep-void beacon in the patrol path of one of the Dark Angels' outrider vessels.

His first acts were to merge many of the teachings of Caliban's techno-feudal aristocracy with those of the I st Legion Hexagrammaton, fusing the best of Terra and Caliban to create something new and more refined, and to gather the scattered fragments of his Legion together. With the first generations of recruits taken from the ranks of the worthy among the knights of Caliban still undergoing implantation of gene-seed, hypnogogic indoctrination and live-fire training, the Lion prepared to embark on a crusade of his own. Perturabo agreed that it was both a prudent choice, and a very humble one. El'Jonson explained that his actions were done for the good of the Imperium, not for accolades, nor for power.Ultimately his better nature won out and he defused the bomb, but there was no way he could hide the truth of his near-betrayal from the Primarch. Luther, and a huge number of other Astartes, were banished to Caliban by Jonson, under the pretence that they were sent there to improve the recruitment of Astartes initiates. He spent no time on parades, fortress-building or in petty squabbles with his kin, but went stoically from battle to battle. He and his Legion began to shun the gatherings of the Great Crusade and the fellowship of their brothers among the Legiones Astartes, scorning those who would fret over such frivolities while there remained enemies of power and strength to test their mettle against. As the years and wars wore on,

Some say the old Legion fought to prove themselves worthy of their new master, others that they bled to make right their failure to destroy the Rangda when first they met at the Battle of Advex-mors, and a few whispered that the Lion sent them into slaughter so he might replace them with more tractable, Calibanite warriors. So it was that when Horus was crowned as the Warmaster of the Imperium the Lion was not present, a victory to the covetous mind of the new I will teach them to fear the darkness in which they dwell, and to dread the shadows they believe their allies, for there is no greater terror hunting the stygian void than the Lion of Caliban." El'Jonson set the knights to ambush the Great Beasts as they came to feed, poisoned the pools at which the creatures drank and set ablaze vast tracts of the forest to set them to flight. He gave the foe no respite and hunted them till no more could be found, and when his warriors spoke of his prowess and victories, it was fear that coloured their words as much as awe. The two formed a close partnership. Jonson had the supernatural tactical acumen engineered into him by the Emperor, while Luther had an easy charisma that the isolated Jonson could never match. Gradually, the two great warriors became famed and respected war leaders, and close friends.

Perturabo inquired of El'Jonson how he had learned of the existence of the Ordinatii siege engines. Jonson explained that he had discovered them fifty Terran years earlier when he was studying the history of the Great Crusade and saw a reference to them in a despatch from Horus that had been sent to the Emperor. Due to the extensive losses suffered by the I st Legion during the Rangdan Xenocides, by 899.M30, the Ultramarines Legion were on the cusp of becoming the largest Space Marine Legion, standing at around 166,000 Legionaries. Even if he were somehow to prevail against Perturabo and the other Legions sent to confront him in the Isstvan System, the Warmaster couldn't claim total victory so long as the Emperor was safe in His palace. For Horus to triumph, the Emperor had to die. And that meant a long and costly siege of Terra. Therefore, the Warmaster would come to claim the siege engines of Diamat. Thanks to the skilled coordination and superb execution of the Lion, the Night Lords fleet was devastated, losing dozens of capital ships and approximately one-quarter of their Legion fleet to the Dark Angels' assault.

A Triumvirate was later held where Curze defended his actions, but refused to admit his guilt. Since each of the primarchs had been created to perform a specific function, Curze argued he was merely acting according to his own nature, and therefore had committed no crimes. When news of Horus' new rank reached Lion El'Jonson he did not pause in his campaigns, nor did he offer congratulations or lament his own fortune and this, more than the reaction of any of his other brother primarchs, gave the Warmaster pause. When Horus' thoughts later turned to rebellion and treachery after his fall to Chaos in the Temple of the Serpent Lodge on Davin, it is likely that it was the Lion he marked as among the greatest of threats to his plans. The Lion could stand against any of his kin, match blades with Fulgrim and stalemate the strategies of Roboute Guilliman and, though some might exceed him in the details of some tasks there were none that were his equal in the grander scope of battle, none whose will could match the bloody-minded determination of the Lion. His talents and resolute confidence, which some might have called arrogance, won him few friends but saw him placed at the head of his Legion faster than any of the primarchs to be rediscovered before him. As for Lion El'Jonson, his brother primarchs would come to call him dour and morose, given to dark moods and heedless of the counsel of others, but he saw things simply and starkly. He learned on Terra that the war he had fought in Caliban's monster-haunted forests had not been ended, but only begun -- for the galaxy teemed with monsters to be slain. He dedicated himself to one task: killing. Johnson (the poet, not the Primarch) was gay, and could not reconcile his sexuality with his Catholic faith, nor with the homophobic society he lived in. The Dark Angel is a poem about his inner struggle.The Sarosi -- ruled by a planetary bureaucracy -- had recently expressed their interest in becoming part of the Imperium, and the Imperials were eager to allow them in, believing that these people seemed to possess the same secular beliefs as they did in the Imperial Truth. But over a standard year had passed and the Sarosi were as yet no closer to attaining Compliance, constantly apologising to the Imperial planetary governor chosen for their world, Harlad Furst, that their bureaucracy was slowing the process. Wary of both sides' motives, especially those of the Death Guard's First Captain Calas Typhon, the Lion prevented the device from falling into the Death Guard's hands. Jonson proceeded to serve his own ambitions and requisitioned the device for his own use. In 008.M31, the Dark Angels received intelligence from an astropathic message from the nearby Perditus System about Traitor movements. They immediately moved to intercept. Upon arrival, they interrupted the month-long conflict between the Iron Hands 98 th Clan-Company, led by Casalir Lorramech and a large Death Guard contingent, led by First Captain Calas Typhon.