The Horus Heresy (M31)

The second most important event in human history is Warmaster Horus' (the Emperor's second in command during the Great Crusade) betrayal of the Emperor. Although Horus would eventually be defeated, the cost to the Imperium was great – fully one half of the Legiones Astartes also rebelled and many other Imperial servants followed their heretical lead. The Emperor ascended the Golden Throne and the High Lords of Terra ruled the Imperium in his name. Many great reforms were undertaken, including the dissolution of the Marine Legions in accordance with the Codex Astartes.

012.M31 Horus Heresy begins.

014.M31 Horus defeated and the Emperor ascends to the Golden Throne of Terra.

Age of the Imperium (M31-M41)

The Age of the Imperium begins with the Emperor's ascension to the Golden Throne and will last eternal. That being said, there have been times when the Imperium has been stronger than at other times.

M31-M32 THE AGE OF REBIRTH: The Imperium rises from the ashes of the Horus heresy under the direction of the High Lords of Terra.

M32-M34 THE FORGING: The Golden Age of the Imperium as the high lords of Terra continue the Emperor's holy work of uniting the galaxy, adding regions untouched even during the Great Crusade.

M34-M35 THE NOVA TERRA INTERREGNUM: The Imperium is divided for nine centuries as Nova Terra, and most of Segmentum Pacificus with it, rejects the teachings of the Ecclesiarchy and the rule of the High Lords of Terra.

975.M35 Nova Terra is dealt with in a most violent fashion, and the Imperium is reunited.

M36 AGE OF APOSTASY: Zeal eclipses reason, and misrule reigns supreme. The word of the Emperor is subverted by demagogues and corrupt officials. Sebastian Thor reforms the Ecclesiarchy and puts an end to this age.

M37 AGE OF REDEMPTION: The sins of apostasy are purged by blood and tears. The Imperial cult grows in power among the masses, and the heretic pyres burn high and bright, day and night. Countless crusades and campaigns are launched, bringing vast tracts of space into Imperial hands. At the dawn of M38 the Imperium is overextended and its seemingly endless might spent.

M38-M41 THE WANING: The High Imperial era is long gone and the Imperium finds itself beset by enemies without, within, and from the other side. The greatest threat is more insidious however, for it is the slow and inevitable stagnation of the Imperium, a leeching of its vital spirit and the onset of cultural necrosis.

744.M41 Taggarath, the Seer of Corrinto proclaims the approach of the End Times. He prophesies a time of unprecedented upheaval and hardship, in which even the light of the Emperor is swallowed by the approaching darkness. Traggarath is executed for treason and heresy, and all his prophesies are purged from public records.

Time of Ending (M41+)

By the later half of M41 there is no denying it; the Imperium's traditional enemies have multiplied and grow strong, and new threats have arisen from the dark reaches of the cosmos – the Orks have grown numerous and bold, young and vigorous xenos Empires like the Tau have been allowed to grow, the Eldar are no longer as zealously prosecuted or even considered allies by some, corruption and Chaos incursions abound, something has awoken the Necrons from their tombs, and from the cold reaches of intergalactic space comes the Great Devourer.

Traggarath was no doubt a traitor and deserved the pyre, but whether you call it the Time of Ending, the Age of War, or the Wolf Time – it is here and you live in it. It will be up to you and those you call brothers to stave of defeat and snatch victory from the jaws of improbability. Now, there are those that actually welcome the Time of Ending; those that do see is as the only way for the Imperium to evolve – to survive is to break free from the stasis that has been slowly strangling the Great Imperium of Man.