The Horus Heresy: Book Four - Conquest is the fourth book released as part of Forge World's Horus Heresy Series of campaign and army books that takes place during the Horus Heresy. The book deals with Horus' invasion of the northern Imperium after his victory at the Isstvan System. The book contains rules and special characters for the Pre-Heresy Solar Auxilia and several Knight households of the Mechanicum. Included within is the Questoris Knight Crusade Army list and the Solar Auxilia Crusade Army list along with the rules needed to play a Battles in the Age of Darkness campaign.