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.

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.

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.

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....

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.

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.

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.

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 Great Crusade that has taken humanity into the stars continues. The Emperor of Mankind has handed the reins of command to his favoured son, the Warmaster Horus. Yet all is not well in the armies of the Imperium. Horus is still battling against the jealousy and resentment of his brother primarchs, and when he is injured in combat on the planet Davin, he must also battle his inner daemon. With all the temptations that Chaos has to offer, can the weakened Horus resist?

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.

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?

After the betrayal at Isstvan, Horus begins his campaign against the Emperor, a galaxy-wide war that can lead only to Terra. But the road to the final confrontation between father and son is a long one - seven years filled with secrecy and silence, plans and foundations being formed across distant stars. An unknown history is about to be unveiled as light is shed on the darkest years of the Horus Heresy, and revelations will surface that will shake the Imperium to its very foundation....

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.

Attached to Crusade Fleet Quintus - dubbed the Cursed Fleet by many - the Ultramarines of the Ithraca’s Vengeance are drawn to a stricken world. With millions enslaved, a malign necron technology siphons the souls of the innocent and heralds the Silent Kingdom’s expansion. The Ultramarines face an impossible decision: mount a desperate last stand to destroy the Pariah Nexus, or break away and damn the entire sector to bring word of this ancient foe’s resurgence to the only being capable of halting it - the Lord Primarch Roboute Guilliman.