From their website:
Horus Heresy is a brand new board game that takes place in the grim darkness of the far future of Warhammer 40,000, one of the preeminent science fiction settings ever created. It is also without a doubt one of the most iconic environments of all time, having remained in active development for more than two decades, across a wide variety of media from tabletop miniatures games, board games, novels and more. The sprawling mythos, baroque aesthetic sensibilities, and dark gothic outlook are unmistakeable, and are enjoyed by legions of fans all around the world.
We at Fantasy Flight Games were honored to have an opportunity to contribute to this richly textured history. We chose to highlight the most pivotal and climactic moment from the Warhammer 40,000 canon, a seminal event that occurred in the Imperium of the 31st millennium. It is a tale that has also fiercely captured the imaginations of writers and fans alike, as it rings with grand, mythic significance.