Hereticus by Dan Abnett

In the finale of a grimdark Inquisitorial trilogy, Inquisitor Gregor Eisenhorn wages a relentless hunt against a sprawling conspiracy of heretics and daemonic influence across the Imperium, confronting betrayal within his own circle and the moral cost of using forbidden means. As the threat escalates, he must balance ruthless pragmatism and personal integrity while racing to stop a plot that could unleash catastrophic chaos.