For all the paths taken and possibilities the Architect of Fate could perceive, he knew that a few moments were crucial. Events that resonated and took the same names no matter what surrounded them.

The Heresy. Issvaan V. The God-Emperor's Internment. Prospero. Nikaea. Ullanor. Laer.
All so simple, in retrospect. The players could change but the play remained the same.

It bored the Architect of Fate, to be so constrained...

But there was another way. Another branch untaken. And as Tzeentch laughed, the strands of fate wove another tale...

-

It is the closing of the forty-first millennia, and there is only war.
Bound onto the Golden Throne by grim necessity and His own commands, the God-Emperor of Mankind lays preserved against the ravages of time and the Great Enemy, His mind and will ceaselessly working to keep His dreams for mankind alive. On the borders the Astra Imperatorus battles without end against xenos empires and other, greater threats. The last of His loyal sons are dead or missing, and have been for countless generations, but the soldiers forged in their image still strive and fight regardless. The Inquisition and their sombre guardians the Night Lords work within, keeping the taint of heresy at bay. The Iron Warriors hold the line around the Great Eye of Terror, their fortresses beyond compare, while the Fallen Angels and Alpha Legion harry the corrupted Ultima Sector, keeping the horrors of that Chaos-consumed realm and its tainted Primarch at bay for eternity. The Desert Fangs, Space Wolves, and World Eaters compete with one another as they battle their enemies, striving to prove whether discipline, honor, or righteous fury is the most potent component of the Astartes. The Emperor's Children, clinging to tradition and aging ideals, seek to crush another race that, in a twisted mirror of their own ideals, seeks perfection as well- the Tyranids. The Crimson Hands, once the Iron Hands, work tirelessly to bring new weapons and wargear into the fray for the Imperium, to preserve it from enemies within and without. The Thousand Sons, broken by the loss of their homeworld to the Death Guard and the plagues of that forsaken Legion's patron, serve as they always have, their libraries preserved away from Nurgle's rot, and seek to find the keys to all knowledge. The Black Legion, once the Sons of Horus, are the sword arm of His Will, ever-hungry for new planets to bring into the Imperial fold, though there are worries that they have been corrupted by contact with xenos and the heresies those benighted races commit.
In the Eye of Terror, the Gods of Chaos play their games, setting Legion against Legion. The Arch-Traitor Sanguinius' corpse lays preserved there for all eternity, a lifeless husk, guarded by the twisted and vampiric remnants of his once-proud Legion. The White Scars seek speed and sensation, the traitorous elements of the Omega Legion absolute perfection, both under the eye of the Bane of Eldar, Slaanesh. The heart-claiming Sky Serpents and the destructive Salamanders follow the Lord of Skulls, Khorne, their Primarchs obsessed with old hatreds and bitter glories. The Death Guard and the 'Imperial' Fists, called the Excoriators by the Imperium they mock, serve Nurgle with stubborn defiance, unwilling to bend to the demands of the Grandfather unless forced. And the orators of the Word Bearers and the secretive Dark Angels serve the Lord of Lies, forever weaving plans within plans within plans. In Ultramar, the Legion that took the planet's name tries to hold together with their Primarch lost to stasis and their worlds forever in disarray, turning to dark arts to swell their numbers. Across the Imperium, working against the Inquisition, lays the last of the Traitors, the bitter and jaded Raven Guard, those who saw hope die and turned to the Warp to ensure that all the galaxy shall feel the same misery...and who, in the end, serve Malal.

The Imperium is dying.
Despite the best efforts of the Crimson Hands and the Mechanicus, the Golden Throne grows more fragile by the day. Despite the heroism of the loyal Legions, the xenos advance. To the east the Tau gather a confederation, and rumors of Sons of Horus detachments among their ranks cause schism and civil war. In the Eye, the traitor Primarchs stir, united, perhaps, under Dante the Lord of Angels, Bringer of Darkness. A Black Crusade with such leaders could finally break the Iron Warriors and cause havoc across the Imperium. The Thousand Sons speak of grim days and horrible visions, and even the Space Wolves speak of storms to come. Across the galaxy, the Eldar scheme, the Orks unite under a new Warboss for the largest WAAGGH! the galaxy has known since the War of the Beast, the Necrons rise from their tombs, and long-forgotten secrets are uncovered once more...
It is the forty-first millennium, and there is only war...but all is not yet lost.

Prior to the Heresy and the War of the Beast, the Rangdan Xenocides were the point at which the Imperium had come closest to destruction. The Rangdan, a horrific race that fed on the brains of living sentients and worshipped what has been postulated to be an aspect of Nurgle, possessed great biological power, similar to the Laer and the later Tyranids. Their eldritch power and hideous biology caused the destruction of multiple Knight Households and Titan Legions deployed against them, and even the power of multiple Legions deployed against them was barely sufficient to secure stalemy. With the Breaking of the Labyrinth of Night and it's subsequent resealing, though, the Rangdan were thrown into disarray, their powersdisrupted and swarms of arcane machinery unleased upon them before vanishing as the Labyrinth was sealed once more. The Astartes and Navy seized the opportunity and crushed the Rangdan without mercy, subjecting the worlds under their thrall to Exterminatus.
The Xenocides mark in their history the one point where an Astartes Legion- indeed, two of them- have come closest to extinction. Upon the world of Pyros III, the Desert Fangs and Sky Serpents had been deployed together to root out a Rangdan infestation. The Sky Serpents, always impetuous, ignored the orders of Kole Rachnus, who had been placed in overall command of the effort. Instead of the measured advance Kole counseled, they rushed ahead of supply lines, and the Rangdan struck with overwhelming force at both them and the Desert Fangs who attempted to rescue their brothers, surrounding them in short order. It was only due to the arrival of the prognosticating Primarch Magnus the Red and his subsequent call for assistance bringing the Emperor Himself and His Custodes into battle that preserved the two Primarchs and their Legions from being butchered on the field.
In addition to these near-losses, the Dark Angels Legion took heavy losses in the battles of the Xenocides, and it is thought by some that the corruption of certain elements of the Legion began in those bloody days as their numbers dwindled, leaving them no longer the strongest of the Legions.