The Chaos Space Marines known as the Fallen Angels are amongst the most enigmatic followers of the Chaos gods, if indeed they are followers at all. Their origin has its roots in the dying days of the Horus Heresy, when Lion El, Jonson, primarch of the Dark Angels Chapter of Space Marines, returned to his home world of Caliban. As the unsuspecting ships of Jonson's fleet moved into orbit they were met by a devastating barrage of defense laser fire. Ships exploded into flame and crashed to the surface like monstrous comets. Stunned by the attack, Jonson withdrew and attempted to find out what had happened to his home world. A captured merchant ship soon provided the answer. When Lion El' Jonson had left Caliban to take part in the Great Crusade, Luther, his second in command and life-long battle-brother, had been left behind in charge of the remainder of the Dark Angels on Caliban. Despite the importance of Luther's position, it was not one that suited his ambitious personality and soon his role, as planetary governor of some half-forgotten backwater world seemed more and more to him like an insult. These seeds of jealousy grew until Luther had become a man obsessed, whose own neuroses had pushed him over the edge and made him dangerous beyond imagining. Using his renowned skill at oratory, Luther convinced the Dark Angels under his command that they had been shamed, that the Emperor had turned his face from them, instilling his own feelings of jealousy and rage in the Dark Angels who had been left on Caliban during the Great Crusade. When the Primarch returned, these feelings of betrayal erupted into open rebellion. The fury of Jonson and the loyal Dark Angels at this terrible betrayal knew no bounds. They had fought from one end of the galaxy to the other and thought that the curse of Chaos had been cleansed from the planets of the Imperium. Now they found that their own home world, and their own brethren, had been corrupted and turned against them. Jonson immediately led an attack on the greatest of the Dark Angels' fortress-monasteries himself. He knew that his was where he would find Luther: and so it was that the two former friends; now mortal enemies, faced each other. Even though the Primarch possessed superhuman powers, the two opponents were equally matched, for Luther's already considerable abilities had been enhanced by the Dark Gods of Chaos. What followed was a fight of titanic proportions during which the two adversaries laid blow for blow against each other, tearing down the monastery around them until the whole massive edifice had been leveled by their battle. Meanwhile the massed guns of the fleet pounded the planet, until the very surface of Caliban began to crack and heave under the strain of the bombardment. As the planet itself started to break apart, the battle between Jonson and Luther reached its climax. Luther, aided by the powers of Chaos, unleashed a furious psychic attack that knocked Jonson to his knees and left him mortally wounded. But as the dying Primarch struggled to stand, his noble features wracked with pain, it was as if a curtain was lifted from Luther's eyes and he realized the full extent of what he had done. His was a triple betrayal: of his friend, of the Dark Angels, and of the Emperor. The truth shattered his sanity and he slumped down beside Jonson, no longer willing to fight. Luther's psychic cry of pain and despair echoed though the warp and the Chaos Gods realized that, once again, they had been defeated. They lashed out in fury and frustration. A rent appeared in the very fabric of space and a warp storm of unprecedented fury engulfed Caliban. In an uncontrollable, swirling flood of psychic energy the warp rushed into the physical universe. Those "fallen" Dark Angels who had served under Luther and his clandestine masters were sucked from the face of Caliban into the warp and scattered throughout space and time. Caliban, already weakened by the loyal Dark Angels' bombardment, was ripped apart and destroyed, the debris being sucked into the warp. This story of treachery and betrayal is the Dark Angels' secret shame. None know of it other than the Dark Angels, their Successor Chapters and, maybe, the Emperor on his Golden Throne. Even within the Chapter itself very few brother-Marines know exactly what happened during those fateful days. It is only when Dark Angels reach the Deathwing that they learn the story of Luther's betrayal. More terrible still they learn that many of the Dark Angels that followed Luther are still alive. These damned warriors are called the Fallen Angels. Not all of the Fallen Angels have succumbed to the power of Chaos to the same degree. A large number of the Fallen have embraced the ways of the Dark Gods totally becoming true Chaos Space Marines. However, many others realize that their actions during the fall of Caliban were wrong. Disgusted by the corrupting influence of the Chaos Gods and unable to reconcile themselves with the Dark Angels they lead a solitary existence. Many become mercenaries or pirates, roaming the galaxy as masterless men. Others are willing to atone for their sins and in an attempt to do so have integrated themselves back into human societies taking on the role of any ordinary person. However, whatever their subsequent actions, in the eyes of the Dark Angels the only way that they can rid themselves totally of their shame, and restore their honor and trust in the Emperor's eyes, is if all the Fallen are found and either made to repent or are slain. This is by no means an easy task. The Fallen are dispersed throughout space and time as either isolated individuals or in small bands, and the Dark Angels can go for years without hearing any rumors that might lead them to one or more of the Fallen. When they do, however, and their mission is a success, those Fallen that are captured are taken back to the Rock. Deep inside its dungeons Interrogator-Chaplains attempt to make the Fallen repent. Occasionally they do and for their pains die quickly. More often than not though, the captured Fallen refuses and suffers a long, drawn-out and agonizing death at the hands of those who would save his soul.
