Perturabo: The Hammer of Olympia

The fourth book in The Horus Heresy: Primarchs. Perturabo, primarch of the Iron Warriors, was hailed as a prodigy on his home world of Olympia, yet he has grown bitter at the thankless work his Legion is given in the Great Crusade. When revolt breaks out on Olympia, he pulls the Iron Warriors away from their campaign against an alien enemy to put down the uprising by whatever means it takes.