The First Heretic by Aaron Dembski-Bowden

Amidst the galaxy-wide war of the Great Crusade, the Emperor castigates the Word Bearers for their worship. Distraught at this judgement, Lorgar and his Legion seek another path while devastating world after world, venting their fury and fervour on the battlefield. Their search for a new purpose leads them to the edge of the material universe, where they meet ancient forces far more powerful than they could have imagined. Having set out to illuminate the Imperium, the corruption of Chaos takes hold and their path to damnation begins. Unbeknownst to the Word Bearers, their quest for truth contains the very roots of heresy...