Fulgrim (The Horus Heresy)
The fifth Horus Heresy novel, and the one that turns. Fulgrim is primarch of the Emperor's Children, a legion built on the pursuit of perfection, and the book follows how that pursuit curdles into excess and then into Slaanesh. The daemon-possessed sword he takes on Laer is the hinge. It ends with the Isstvan V drop site massacre, and it is generally regarded as the point where the Heresy series stops being a war story.