 The majority of stories in "The Horus Heresy" are set around the start of the fictional universe's 31st millennium, in contrast to most "Warhammer 40,000" material, which takes place in the 41st millennium.The stories in "The Horus Heresy" routinely cover the conflict's background, with some going back centuries before the war (and the 31st millennium) begins.In narrower scope, the entire conflict (including the Heresy proper and its proximate formative period), is depicted as having lasted less than a decade.