So, introductions. I've posted this in the 'Alien Space Bats' section of alternatehistory.com, where it was met with positive response, so I thought, why not post it here, with a wider audience of 40K fluff fans? My primary inspiration is, of course, the Dornian Heresy, but there are other inspirations of course...
Chapter One: Ullanor
BLOOD SAMPLE: Match.
WELCOME TO THE CUSTODIAL ARCHIVE, CAPTAIN-GENERAL
Taken From The Great Crusade: A History (Remembrancer Stefana Castellucio)
"Ullanor. That was where Father made his greatest mistake." - Horus Lupercal
The Ullanor Campaign was the greatest of the Emperor's campaigns in the Great Crusade, His finest hour. What a pity that it came so soon before the Heresy began, for it was an hour of unparalleled courage and skill of arms from all 18 Space Marine Legions, save for the Sons of Horus, whose fleet was becalmed in the Warp for the duration of the Ullanor Campaign. The Ork Empire of Ullanor was the terror of the Segmentum, led by an exceptionally strong and cunning Warlord who was possessed of impossible might. The Emperor delegated Sanguinius to the position of leading the assault on the main planet, together with his Blood Angels Legion. Leman Russ was to aid in the main attack, while Constantin Valdor, Magnus the Red, and Alpharius commanded the reserve groups. Ten million troops of the Imperial Army - Byzant Janizars, Merican Stormtroopers and many other regiments - were also allotted to Ullanor proper, along with elements of the Mechanicum.
The Battle of Ullanor swiftly became a meatgrinder. Hills, mountains of Ork dead rose swiftly, but yet more and more greenskins rose from foul warrens underneath the planet's surface. The strategies of Magnus and Alpharius proved to be a capable multiplier of force, but the Blood Angels' tactic of throwing themselves forward even in the face of impossible odds, led by their 'Angel' (as he was known then) Sanguinius brought them severe losses. Then, the Emperor descended. How to describe His descent? The grey skies opened up and rained fire upon the Ork hordes like a God's wrath, turning mountains crawling with the vermin into glass plains. Then He unleashed his power, aided by Primarch Magnus. Millions of Orks died in moments, as white fire consumed them from within. The Ork Warlord then rose, a vast specimen, his skin so dark green it was almost black. The monstrosity was the size of a Dreadnought, and massacred the Custodes squads sent forth.
The Emperor Himself arose and did battle with the abomination. The Ork, in a moment of power, tried to crush the Emperor's neck with his power-claw, but then Sanguinius descended. Against Sanguinius and the Emperor both, the Ork could not stand, and was rended limb from limb. That marked the end of the Ork Empire upon Ullanor. The green vermin were scattered, laid low and ultimately wiped out from the planet. The Mechanicum's geoformers then descended, turning the planet's main continent into a glassed plain, fitting for the Triumph that followed. Mysteriously, the Emperor announced during the height of the ceremony that He was leaving for Terra to continue his researches. However, to continue the Great Crusade He delegated command over the Imperial armies involved in the Great Crusade to Primarch, now Warmaster, Sanguinius.
Why he did so is unknown, considering Sanguinius' betrayal and the long shadow it has left on the Imperium. Perhaps He saw a mirror of Himself in the angelic being. As Horus thrice-Praised himself said, Sanguinius had all the measure of the Emperor. Yet even now, as we struggle to both drive the Fallen Legions from the ordered areas of our galaxy and fend off the traitorous Supremacy of Ultramar, it can only be seen as a disastrous mistake. Perhaps Horus would have fared better, yet it is not the time to discuss what-ifs, could-have-beens and never-weres. What we are left with is the world we made, and we must all dwell within.
Taken from the Chronica Prospero, Timestamp 013.M31
...Great slaughter. Russ' and Jaghatai's sons have made the Silver Lake red and black with blood and ink. Magnus is on Terra, with the Emperor, but I can sense his presence with me. The Warp-spawn assault us daily, but we are holding. Our fortress stands yet. But what concerns me is the howling outside, and a terrifying presence at the corners of my psychic perception, like the Warp-spawn but far worse. The Space Wolves are degrading into beasts like the savages they are. They cannot breach our defences as of yet. But what will happen, once Russ himself joins the fray?...
- Ahzek Ahriman, 1st Fellowship Captain
 Initial point of divergence.
 Major point of divergence.
 The text was written during the Reclamation Wars in-universe.
 Will be explained later.
 A bit of irony on my part.
 Later updates will explain this bit.