During the Great Heresy, the Emperor's Children tragically fell, yet some of their members remained loyal to the Imperium. Before the Imperium's extermination order destroyed the Emperor's Children's homeworld of Chemos, a small group rescued a trove of precious supplies from the Legion's fortress-monastery and used them to establish a new Chapter. They firmly believed: "We are the Emperor's children. Our shameful failure betrayed His hopes, but we will never give up. The embers will blaze again, and from the ashes we will be reborn!"
Warhammer fanfiction is hard to write, so I've added some elements that diverge from canon. GW retcons things so often, too, so if the plot or setting differs from what you know, please go by the version in this book.
P.S. The content of the first thirteen chapters has been moved to the supplementary section. The related events are not essential to following the story.