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.

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?

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?

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....

The planet of Caliban exists much as it has for thousands of years - the knightly orders protect the common people, fighting back the beasts that lurk in the depths of the seemingly endless forests. Young Zahariel and Nemiel aspire to join the greatest of the orders, led by the example of mighty Lion El'Jonson and his vision of a peaceful and unified world. But the coming of the Imperium brings new concerns and a new destiny for the Lion as part of the Great Crusade, and the sons of Caliban must decide if they will follow him to glory among the stars.

Now that news of Horus' treachery is in the open, a time of testing has come. Some Legions have already declared their allegiance to the Warmaster while the loyalty of others lies firmly with the Emperor. As Horus deploys his forces, loyalist Astartes learn that a massive Wordbearers fleet is heading to Ultramar, home of the Ultramarines. Unless they can intercept the fleet, and destroy the mighty battleship that has been sent to reinforce it, the Ultramarines may suffer a blow from which they will never recover.

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.

Censured at the Council of Nikaea for his flagrant use of sorcery, Magnus the Red and his Thousand Sons Legion retreat to their homeworld of Prospero to continue their use of the arcane arts in secret. But when the ill-fated primarch forsees the treachery of Warmaster Horus and warns the Emperor with the very powers he was forbidden to use, the Master of Mankind dispatches fellow primarch Leman Russ to attack Prospero itself.

When Horus the Warmaster rebelled against the Emperor, the ensuing civil war nearly destroyed the Imperium. War raged across galaxy, pitting Astartes against their battle-brothers in a struggle where death was the only victor. But long before that, the Legions struggled with their identities and the demands of the Great Crusade. From battles with alien invaders to conflicts within a Legion, and even dealing with a wayward primarch, these stories show the battles fought as the Great Crusade draws to a close and the Horus Heresy begins....

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.

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.

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.

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.

In the void and upon the worlds of Greater Ultramar, the battle for the Imperium continues. Intent on rebuilding his home realm and using it as a base to reconstruct the ravaged stellar empire of mankind, the returned primarch, Roboute Guilliman, proceeds with his war to drive Mortarion and his Death Guard Traitor Legion from the domain of the Ultramarines. But when Guilliman brings his brother to battle upon the diseased plains of Parmenio, the intervention of a greater power in their fraternal struggle threatens to upend the Imperial Regent’s understanding of the galaxy. 

After a series of victories, the Imperial forces are on the back foot once more. The power of Chaos is rising, the Traitors gain ground, and all hope seems lost. Can Terra endure?