Perturabo

Perturabo was a primarch of towering intellect and towering resentment, a mind of unmatched genius for engineering, mathematics, and the science of destruction, wedded to a soul that believed itself forever slighted. Recovered from the city-states of Olympia, he raised his adopted world to greatness and then, in a single campaign of terrible fury, destroyed it, crushing an Olympian rebellion so brutally that the act stained the IV Legion forever with the guilt of their homeworld's ruin.

He led the Iron Warriors as masters of the siege, the Legion the Imperium called upon to break the unbreakable and then left unthanked, spending them on grinding attrition-work while their brothers claimed the glory. That grievance, nursed across the long decades of the Great Crusade, curdled into a bitterness Horus knew exactly how to exploit. When the Warmaster rebelled, Perturabo brought his Legion to the traitors' banner, less out of love for Horus than out of a cold conviction that the Imperium had never valued what the Iron Warriors were. Behind his adamantium shield-guard he carried his resentment into the Heresy, a genius of destruction at last unleashed against the very civilisation his labours had built.