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.

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.

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?

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.

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.

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.

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 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.

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 galaxy is burning. The Emperor's loyal primarchs prepare to do battle with Warmaster Horus and his turncoat Legions on the black sand of Isstvan. Such dark times herald new and yet more terrible things still to come, and when Astropath Kai Zulane unwittingly learns a secret that threatens to tip the balance of the war, he is forced to flee for his life. Alongside a mysterious band of renegades, he plunges into the deadly underworld of Terra itself, hunted like a criminal by those he once trusted.

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.

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.

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?

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.