Horus Heresy: Before the Heresy, Horus was regarded as one of the military greats of the Imperium. But despite being given the honourable title of Imperial Warmaster, Horus was possessed by a daemon and turned to Chaos by a warrior cult on a feral world.
The recruitmen of the heretics occured during the Imperial Crusade, when Horus created Warrior Lodges in the legions under his command in a mirror of the warrior cult that had corrupted him. The rot had also spread to associated Adeptus Mechanicus, Collegia Titanica and Legio Cybernetica under his command.
The Imperium was not first awakend to Horus' treachery until Istavaan when a mission to quell a Commander's rebellion lead to the planet being destroyed and loyalists escaping in a single frigate to warn the Emperor. Many see this as the start of the Inter-Legionary Wars.
The Emperor sent the Inquisition to purge the Adeptus Mechanicus, but the Order began in-fighting as some joined Horus while others stayed loyal. The Titan Legions on Terra remained loyal to the Emperor, and the Colledia Titanica supply depots remained loyalist. This led the traitor Titans to attack before they were crippled through lack of supplies. This attack worked, and the traitors captured enough supplies to make use of their Titans in the final attack on Terra.
As every section of the Imperial military was pulled into the heresy, a full third of the Legions Astartes were recalled to destroy Horus. The Emperor took 180 days to plan and launch his attack against Horus, during which time the rebel Warmaster had spread his claim as the 'New Emperor'.
After Horus had captured Istavaan when he was sent to return it to Imperial control, the Emperor sent seven full legions of Space Marines to return the planet. The first assaults by three legions were annihilated, leaving just five marines bearing the geneseed of their chapters. The remaining four chapters were converted to Horus' cause.
Horus then headed for Terra, planning to destroy the Emperor and take the Imperium as his own. Within 30 days his troops were entrenched around the Imperial Palace and were fighting to a stand still. All that remained to oppose them was the Adeptus Custodes, the Imperial Fists and Whitescar Space Marines and the loyalist Collegia Titanica. By the 55th day, though, the Traitor Legions had reached the inner walls of the Imperial Palace.
When the Emperor was forced to abbandon the outer palace, he took action and teleported to Horus' battle barge. The Blood Angel's Primarch, Sanguinius, found Horus first and his broken broken body was found by The Emperor when he entered Horus' bridge. In an epic battle the Emperor defeated Horus, but was severly wounded in the process.
With the death of their leader, the rebels paused in their assault and then fled, chased back to the Eye of Terror. Meanwhile the Emperor was put within a life-sustaining bubble while the Golden Throne was created.
The Emperor declared Horus and his legions Traitor and had them confined to the Eye of Terror, with all record of them being removed or marked Excommunicate Traitoris. Fighting continued in outlying areas for seven more years, but the traitors were eventually driven to the Eye of Terror or underground.
It would take many changes and a long time for the Imperium to return to its former strength, but under the guidance of the Emperor in his Golden Throne and with the ever more vigilent eye of the Inquisition watching over it the Imperium was rebuilt, awaiting the day it would face the forces of Chaos again.
Source: Unknown by unknown
Submitter: IBBoard
Hive World Terra