Abyssal Angels

The Abyssal Angels are a space marine chapter loosely formed during The Horus Heresy. It was initially formed from broken companies of the Luna Wolves and deserters of the Sons of Horus. 
During the time of The Heresy, most Luna Wolves followed the Brightest Star, their primarch Horus Lupercal, as he led a righteous rebellion against the Emperor of Mankind, who was revealed to have hypocritically spun lies about the Imperial Truth of the galaxy and virtually enslaved the Astartes to blindly fight His Great Crusade. However, others saw the ruinous destiny in this rebellion, the fall of the glorious reforging of mankind's galactic empire, and refused to stay with the newly dubbed Sons of Horus legion. Some of these found new purpose in The Knights-Errant, a newfounded and solemn legion of orphans dedicated to the protection of Terra from Horus' impending siege. Yet others drifted outwards, into the black abyss of space, neither prepared to rebel against the Empire, nor willing to fight for the Emperor.
The Abyssal Angels were initially primarily warriors of a Luna Wolves company, who were on a mission of their own far away from Isstvan during the terrible event of that system. Months had passed since the massacre before they received word of it. They could scarcely believe it, had it not been delivered from Luna Wolves battle-brothers they knew and trusted, who had fled from that dread site. 
Upon the news that Horus had taken up arms against his father, they found themselves in a confusing situation where countering oaths forced them to inaction. Upon completion of their mission, they were to rendezvous with other companies, but the nearest companies had gone to Isstvan, and how could they rejoin those forces? 
Later, they were joined by deserters of the Sons of Horus. Space marines who initially stood by Horus in his choice, but subsequently railed against the actions they found themselves carrying out. The massacre at Isstvan and the destruction of Molech were two such actions, after which some Sons of Horus found their way out of the Legion and into the refuge of the empty reaches of the galaxy, later to find and join forces with the Luna Wolves company that would become the Abyssal Angels.
As such, the Abyssal Angels see the wisdom in breaking free from the Imperium, but will also not surrender to the chaos gods, and so fight for their own ideals. 
They had been sent to bring compliance to a world, as all expeditionary fleets of the Great Crusade were. When they received the news of the unfolding galactic civil war, they chose to fortify the latest system that they had made compliant. They now protect it as their base in the galaxy, and they have renamed it Bastion. From Bastion, they try to conquer and rebuild more of the galaxy in the name of reason.