"The Emperor gave them reason... He brought them into the light. And in scarcely a thousand years they spat in his face and willingly crawled back into the dark. So be it, as Astartes, we will bare the torch, and shoulder the blame of persecution. We will drag them, kicking and screaming, into the light. As the Emperor commanded then in the days of the Great Crusade... So he would surely command now."
― Supreme Grand Master Gilliam the Lightbringer
Once a fiercely loyal Chapter of the Dark Angel's Unforgiven, the Fire Lions turned from the Imperium they had sworn to uphold during the dark days of the Nova Terra Interregnum, supporting those who opposed the establishment of a theocratic state and succession from the Imperium and its new Imperial Cult. Supporters of the Emperor's Imperial Truth, the Fire Lions had become disgusted with the Imperium as it devolved into a brutal theocracy, and joined the ranks of the Fallen as they threw their loyalty and considerable might behind the Ur-Council of Nova Terra.
In the resultant bloody civil war, the Fire Lions further compounded their sin with efforts of Legion Building and procuring archaeotech from various loyalist Adeptus Mechanicus Forge Worlds. By the time the Interregnum came to a close, the Fire Lions numbered easily over 1,500, known by the separatists as the "Legion of Nova Terra" or "The Legion of Reason". However, after the Ur-Council crumbled, the Fire Lions were forced to flee with scores of refugee fleets to the very borders of Segementum Pacificus. It is said by Imperial scholars that the Fire Lions still reside in the deep space at the edge of the Galaxy, having formed a small fleet based nation-state based on the edicts of the Imperial Truth. Now little more than vagabonds and pirates, the Fire Lions seek to rekindle a rebirth in the Imperium's golden age of reason through brutal terror strikes and by seeding the stars with agents of doubt. Only time will tell if these elusive agents of disbelief will continue to evade their former kin within the Unforgiven and the ever vigilant eye of the Inquisition.