Fulgrim

This book by Graham McNeill is the fifth Horus Heresy instalment. In this one, we again skip backwards a few years to follow the strands of story around Fulgrim and his legion, the Emperor's Children - who pride themselves on being the legion that most strives for, and in large part attains, perfection in all they do. Like the Luna Wolves had before, the legion has a cohort of civilian remembrancers attached to their crusade fleet to record the great deeds achieved in the war to reunite humanity. The aesthetic experience in a temple-like construct of a defeated alien race called the laer seems to have a profound and disturbing effect on many of the legion and the remembrancers alike; this influence builds slowly over the course of this novel into a horrifying excess of expression and enjoyment. But before that peak is reached, Fulgrim is brought into the confidence of Horus's new direction and sides with the Warmaster - then, given the task of persuading Ferrus Manus, primarch of the Iron Hands legion, to join the cause also, Fulgrim tries his best but ultimately fails. This uncloseable breach between the brothers opened, Fulgrim draws the Iron Hands to Isstvan V - where, just mere days after the terribly scenes of Isstvan III, the civil war among the legions rears its head once more in all-out slaughter - including the death of Ferrus Manus himself (the first primarch to die). Despite having been forewarned by an eldar farseer earlier in the book, Fulgrim is by this point so full of pride and surety that even Horus is looking at him askance by the end: and rightly, as those feelings are not the only things Fulgrim is full of - he has been completely consumed and possessed by a daemon of the warp. His life is no longer his own - he has become a passive instrument of the dark powers. So while the chief instigator of the titular heresy is of course Horus, arguably Fulgrim fell further faster.* Quite the trajectory for one book, but trust me, it's well-paced enough that not of it feels rushed nor anything less than inevitable.

* There are a handful of other traitor primarchs who were well underway with their downward spirals before Horus too (especially looking at you, Lorgar), but we'll get to them in turn. This is a long series, folks.