From the introduction:
It is a time of legend.
The galaxy is in flames. The Emperor's glorious vision for humanity is in ruins.
His favoured son, Horus, has turned from his father's light and embraced Chaos.
His armies, the mighty and redoubtable Space Marines, are locked in a brutal civil war.
Once, these ultimate warriors fought side by side as brothers, protecting the galaxy and bringing mankind back into the Emperor's light. Now they are divided.
Some remain loyal to the Emperor, whilst others have sided with the Warmaster.
Pre-eminent amongst them, the leaders of their thousands- strong Legions are the Primarchs. Magnificent, superhuman beings, they are the crowning achievement of the Emperor's genetic science.
Thrust into battle against one another, victory is uncertain for either side.
Worlds are burning. At Isstvan V, Horus dealt a vicious blow and three loyal Legions were all but destroyed. War was begun, a conflict that will engulf all mankind in fire. Treachery and betrayal have usurped honour and nobility. Assassins lurk in every shadow. Armies are gathering. All must choose a side or die.
Horus musters his armada, Terra itself the object of his wrath. Seated upon the Golden Throne, the Emperor waits for his wayward son to return. But his true enemy is Chaos, a primordial force that seeks to enslave mankind to its capricious whims.
The screams of the innocent and the pleas of the righteous resound to the cruel laughter of Dark Gods. Suffering and damnation await all should the Emperor fail and the war be lost.
The age of knowledge and enlightenment has ended.
The Age of Darkness has begun.