Amidst the galaxy-wide war of the Great Crusade, the Emperor castigates the Word Bearers for their worship. Distraught at this judgement, Lorgar and his Legion seek another path while devastating world after world, venting their fury and fervour on the battlefield. Their search for a new purpose leads them to the edge of the material universe, where they meet ancient forces far more powerful than they could have imagined.

After the horrors of Istvaan V, Horus declares outright war against the Imperium. In the shadows of the Emperor's Palace, powerful figures convene. Their plan is to send a team of assassins to execute the arch-traitor Horus and end the war for the galaxy of mankind before it's even begun. But what they cannot know is that another assassin is abroad already, with his sights firmly set on killing the Emperor.

The Emperor is enraged. Primarch Magnus the Red of the Thousand Sons Legion has made a terrible mistake that endangers the very safety of Terra. With no other choice, the Emperor charges Leman Russ, Primarch of the Space Wolves, with the apprehension of his brother from the Thousand Sons home world of Prospero.

Under the command of the newly appointed Warmaster Horus, the Great Crusade continues. Fulgrim, Primarch of the Emperor's Children, leads his warriors into battle against a vile alien foe, unaware of the darker forces that have already set their sights upon the Imperium of Man. Loyalties are tested and every murderous whim indulged as the Emperor's Children take their first steps down the road to true corruption - a road that will ultimately lead them to the killing fields of Isstvan V....

With news of Horus' treachery spreading across the galaxy, the Great Crusade grinds to a halt as the primarchs and their Legions decide where their loyalty lies - with the Emperor or with the rebel Warmaster. The Dark Angels, too, face a time of testing, both among the stars and on their home world, Caliban. Luther, once Lion El'Jonson's trusted second-in-command, now languishes as an exile in all but name while his master struggles to thwart the traitors' advance upon the forge world Diamat.

A Great War is coming, and it will engulf the Imperium of Man. The Space Marines of the Alpha Legion, the last and most secretive of all the Astartes brotherhoods, arrive on a heathen world to support the Imperial Army in a pacification campaign against strange and uncanny forces. But what drives the Alpha Legion?

It is the 31st millennium. Under the benevolent leadership of the Immortal Emperor, the Imperium of Man hasstretched out across the galaxy. It is a golden age of discovery and conquest. But now, on the eve of victory, the Emperor leaves the front lines, entrusting the great crusade to his favourite son, Horus. Promoted to Warmaster, can the idealistic Horus carry out the Emperor's grand plan, or will this promotion sow the seeds of heresy amongst his brothers?

Having witnessed the terrible massacre of Imperial forces on Isstvan III, Death Guard Captain Garro seizes a ship and sets a course for Terra to warn the Emperor of Horus' treachery. But when the fleeing Eisenstein is damaged by enemy fire, it becomes stranded in the warp - the realm of the Dark Powers. Can Garro and his men survive the depredations of Chaos and get his warning to the Emperor before Horus' plans reach fruition?

The Shadow Crusade has begun. While the Ultramarines reel from Kor Phaeron's surprise attack on Calth, Lorgar and the rest of the Word Bearers strike deep into the realm of Ultramar. Their unlikely allies, Angron and the World Eaters, continue to ravage each new system they come across - upon the garrison planet of Armatura, this relentless savagery may finally prove to be their undoing. Worlds will burn, Legions will clash and a primarch will fall.

As the Horus Heresy divides the Imperium, Corax and his few remaining Raven Guard escape the massacre at Isstvan V. Tending to their wounds, the bloodied Space Marines endeavour to replenish their numbers and return to the fray, taking the fight to the traitor Warmaster.

As the flames of treachery spread throughout the Imperium, Horus plots to subvert or destroy all those who would stand against him. On Mars, home world of the Mechanicum priesthood, the great manufactory-cities have long produced much of the weaponry required for the expeditionary fleets across the galaxy - making the world invaluable to whoever controls it in the coming war.

After seven years of bitter war, the end has come at last for the conflict known infamously as the Horus Heresy. Terra now lies within the Warmaster’s sights, the Throneworld and the seat of his father’s rule. Horus’ desire is nothing less than the death of the Emperor of Mankind and the utter subjugation of the Imperium. He has become the ascendant vessel of Chaos and amassed a terrible army with which to enact his will and vengeance. But the way to the Throne will be hard as the primarch Rogal Dorn, the Praetorian and protector of Terra, marshals the defences.

Since the earliest days of the Great Crusade, Sanguinius - angelic primarch of the IXth Legion - was ever among the closest and most loyal of Horus' brothers. But the Blood Angels have long kept their true nature hidden from the rest of the Imperium, and when the Warmaster hints that the key to their salvation may lie in the ruins of a conquered world, the sons of Sanguinius race to claim it.

Unaware of the wider Heresy and following the Warmaster's increasingly cryptic orders, Roboute Guilliman returns to Ultramar to muster his Legion for war against the orks massing in the Veridian system. Without warning, their supposed allies in the Word Bearers Legion launch a devastating invasion of Calth, scattering the Ultramarines fleet and slaughtering all who stand in their way. This confirms the worst scenario Guilliman can imagine - Lorgar means to settle their bitter rivalry once and for all.

Having recovered from his grievous injuries, Warmaster Horus leads the triumphant Imperial forces against the rebel world of Isstvan III. Though the rebels are swiftly crushed, Horus' treachery is finally revealed when the planet is razed by virus bombs and Space Marines turn on their battle brothers in the most bitter struggle imaginable.