The Horus Heresy is a cornerstone event in the far future fictional universe created by Games Workshop as the campaign setting for its Warhammer 40,000 miniatures wargame. It is a relatively short but devastating galaxy-spanning civil war that engulfs the nascent galactic empire of the Imperium of Man early in the 31st millennium of the universe timeline; the war is a major cause of the science fantasy universe's dystopian environment. Initially described in wargame supplements published in 1988, and used as the setting for a 1989 Epic-branded tabletop miniatures wargame, it has been utilised since as background for several of Game Workshop's products.
The Horus Heresy has expanded in scope and standing beyond its original wargame-background role. Since the concept's introduction there has been an increasing number of related releases in several product categories and media. It is an ongoing project that has developed into a major, definitive component of the shared universe, and it has come into its own as a successful brand and product line for Games Workshop and its affiliates. In 2012, it was repositioned as a foreground element in the Warhammer 40,000 gaming system, thanks to the introduction of the stand-alone Horus Heresy miniatures wargame expansion.