Editing Medhammer 40,000 Primarchs (section)

===During the Great Treachery===
It seems that Horus was destined by the Chaos gods to lead the rebellion against the Emperor, but the Primarch refused to transgress his father. The Ruinous powers, not one to take refusal lightly ensured that the Warmaster would be sufficiently punished for his sins against him. The majority of the Treachery, Horus was tortured humiliated, and helpless. The warmaster was brought low and forced to watch his brothers struggle without his leadership, while all of his work was undone.

====Isstivan and The Great Treachery====
What was supposed to be a swift end to what he percieved to be an Uprising of Gulliman and Magnus against the Council of Nikea turned out to be Horus' greatest humiliation. Hearing from his trusted sources in the Alpha Legion, Horus learned that The Ultramarines, The Lion's Dark Angels, Death Guard, White Scars, and Thousand Sons were amassing against the Imperium. Horus brought his own army, the Alpha Legion, The Iron Warriors, Raven Guard, Emperor's Children, and numerous Imperial Guardsmen as Auxilaries to Isstivan IV. The Plan was to ambush the armies and coordinate with the Word Bearers and World Eaters legions, who would join in the fight to surround the Rebellious forces. Additionally, Horus prepared numerous traps, minefields and fortifications (courtesy of the Iron Warriors) to end the rebellion in a swift and brutal stroke.

But it was a lie. The Word Bearers, World Eaters, Alpha Legion, and Iron Warriors were all collaborating with each other. The loyalist elements of the Dark Angels, were imprisoned on Calliban along with their Primarch. The White Scars and Thousand Sons were trapped in the warps storms that engulfed Prospero as the Space Wolves fell to Khorne. Horus was unwittingly walking into a trap Horus Aximand and Lorgar had prepared for him. 

Instead of a quick descisive battle, what happened instead was a bloodbath. The Sons of Horus, Iron Hands, and The Emperors Children found themselves surrounded by traitor legions and never before seen hosts of demonic forces. His imperial guard, broken and unprepared, flung themselves into Horus' own minefields and fortifications hoping that the Emperor's peace would free them from the horrific nightmares bearing witness on the battlefield. The Emperor's Children legion was almost completely destroyed, and many of Horus' most noble sons lie dead before him. Horus himself lost his Left Arm in his fight against Angron. Horus and the others were forced to teleport to their ships, leaving millions of Guardsmen at best to die. 

====Aftermath====
In grim conclusion to such a harrowing defeat, Horus returned to his ship, only to learn that it was his own son, Horus Aximand who had betrayed him, leading most of the Mournival in mutiny against Horus. Horus Aximand, now imbued with the power of the Dark gods and referring to himself as Horus Optimates. Defaced Lupercal, tortured him, flaying his skin, and removing his limbs. Lupercal watched as the Segmentum Obscurus and Pacificus was subdued by Horus, unraveling decades of work. It wasn't until Sanguinius and Jaghatai Khan led their daring assault of the Vengeful Spirit that Horus was freed from the clutches of the his treacherous sons, and only then as a bargaining chip and hostage for their escape.