Age of Darkness by Christian Z. Dunn

After the betrayal at Isstvan, Horus begins his campaign against the Emperor, a galaxy-wide war that can lead only to Terra. But the road to the final confrontation between father and son is a long one – seven years filled with secrecy and silence, plans and foundations being formed across distant stars. An unknown history is about to be unveiled as light is shed on the darkest years of the Horus Heresy, and revelations will surface that will shake the Imperium to its very foundation... Rules of Engagement - Graham McNeill https://www.goodreads.com/book/show/22599254-rules-of-engagement?from_search=true Liar's Due - James Swallow https://www.goodreads.com/book/show/22599267-liar-s-due?ac=1&from_search=true Forgotten Sons - Nick Kyme https://www.goodreads.com/book/show/22599275-forgotten-sons?ac=1&from_search=true The Last Remembrancer - John French https://www.goodreads.com/book/show/22599247-the-last-remembrancer?ac=1&from_search=true Rebirth - Chris Wraight https://www.goodreads.com/book/show/22599263-rebirth?ac=1&from_search=true The Face of Treachery - Gav Thorpe https://www.goodreads.com/book/show/22599271-the-face-of-treachery?ac=1&from_search=true Little Horus - Dan Abnett https://www.goodreads.com/book/show/22599235-little-horus?ac=1&from_search=true The Iron Within - Rob Sanders https://www.goodreads.com/book/show/22599273-the-iron-within?from_search=true Savage Weapons - Aaron Dembski-Bowden https://www.goodreads.com/book/show/22599241-savage-weapons?ac=1&from_search=true