Fulgrim by Graham McNeill

A tragic, character-driven account of a brilliant primarch whose obsessive pursuit of aesthetic and martial perfection slowly corrodes his judgment and soul; seduced by forbidden art, excess and the insidious promises of Chaos—particularly the hedonistic influence that will become Slaanesh—he leads his legion from disciplined excellence into decadence, betrayal and cataclysm, with devastating consequences for himself, his followers and the wider Imperium.