Not a Primarch (like Malcador), but still technically part of this series. Will cover Constantin Valdor's role in the Unification Wars, and according to previews it will hold some new insights on the Emperor's plans.

As it turns out, it doesn't really tell us anything that we didn't know already, though it does expand on a few things. The book is set near the end of the Unification Wars on Terra. The new Provost Marshal, Uwoma Kandawire, has uncovered evidence of some shady doings at Mount Ararat and confronts Constantin Valdor as to the Custodians’ role in that battle. Along the way, he tells her of the war against the warp-tainted Confederacy of Maulland Sen, where the inherent instability of the Thunder Warriors first became apparent. As it turns out, they weren't just genetically unstable; the influence of the Warp also caused them to go more berserk than usual, so it became evident to the Emperor that a long-term solution would be required. Valdor also tells Kandawire about the primarchs being scattered by the Chaos gods; the psychic backlash from the event was so strong that it wrecked a large section of the Imperial Dungeon and killed thousands of those present. Valdor himself waded in to save the stored gene-seed from being destroyed, alongside Amar Astarte, the Imperium’s best gene-wright and the namesake of the Adeptus Astartes, though everyone believed that the primarchs had been killed. The Provost Marshal concludes that the Custodes are trying to make a grab for power and leads an uprising alongside Lord Ushotan, the “primarch” of the Thunder Warriors’ Fourth Legion, who survived the purge at Ararat. Valdor confronts Kandawire and Ushotan outside the Lion’s Gate and explains himself thus: the Custodians and the Emperor are the architects of humanity’s future, and any crime can be forgiven and any virtue dismissed if it is in service to that future. Then he unleashes the fledgling I Legion to destroy the insurrectionists and personally kills Ushotan in a duel. In the aftermath, he explains to Kandawire the Imperium’s ultimate aim: not just Unity on Earth, but Unity throughout the galaxy, a vast undertaking which will require hundreds of thousands of these new soldiers. Meanwhile, Amar Astarte has come to the conclusion that the Space Marine project will fall apart without the primarchs and has decided to destroy the stored gene-seed in order to stop them from failing like the Thunder Warriors did. She manages to blow up the gene-seed vaults underneath the Palace, but it turns out that Malcador already had copies of all twenty batches moved to Luna. He also reveals to Valdor that the Emperor believes the primarchs are still alive and intends to seek them out. Valdor wonders if it wouldn't just be better to abandon them or destroy them outright, since they might be tainted by whatever power snatched them away in the first place. Malcador's dialogue heavily implies that the Emperor actually did have some paternal affection for the primarchs at this point, as he mentions that the Emperor has started referring to them as his sons and suggests that he has a lingering attachment to them which has yet to fade. Valdor's response is equally telling; he notes that the Emperor's "human sentiments" are slowly ebbing away, and Malcador acknowledges that this is the price the Emperor was willing to pay to secure his dream of Unity.