Hidden in the Maelstrom, a philosopher broods. Once, she was the inquisitor Elith Merihem. She led pogroms against the unfaithful and investigated daemon-cults in the bowels of hive cities. Her word carried the weight of the Emperor. She led armies of Adeptus Astartes in battle, and condemned worlds to the fire of exterminatus.
As she grew older and wiser, the metaphysics of the Warp intrigued her. Developing a Xanthist world-view, Merihem concluded that chaos in itself wasn’t evil. Daemons could be enslaved, or even bartered with, all for the good of Mankind. Such research could of course not go unnoticed forever. Eventually her own order let loose the proverbial hounds. Merihem took refuge with Yawgdiel Kagat, chapter master of the Sol Invictus, who shared her convictions. For all their might, the Sol Invictus were powerless against inquisitorial persecution, and fled into the Maelstrom.
Now Ilith Merihem is known as the immortal Grand Inquisitor. Yawgdiel Kagat and the Sol Invictus have forged an empire for her, bringing the light of liberation and reason to nearly a hundred worlds, governed by that age-old adage: ”Do what thou wilt.”
She is a bloated and toad-like figure, spiritual leader of a revolutionary movement, teaching that individuals are free to choose for themselves. No gods can govern over Mankind – not even the gods of chaos. They and their servants should be neither reviled nor revered. To the illuminated, they are just alien entities whose powers can be carefully used. Ultimately, her goal is to resurrect the Emperor as a fifth great chaos power, and in doing so, topple the immoral dictatorship of the Imperium.
There, a bit more background for the Sol Invictus. To be ret-conned as needed!