Disclaimer: I do not own any franchise used in this story
AN: Alright, this was how I pictured the end of the Imperium building up. So this is in fact the prequel to the last chapter. I wasn't planning on putting it up, but there were enough statements about canon that I will.
Man was a beast like no other. Intelligent, numerous, strong. All these words could be used to describe them. Bloodthirsty, lusting, chaotic, diseased. These words too could describe that great beast that was man. He was his own salvation and damnation. In the grim darkness of space he relied only on his own strength and that of his brothers. But it was not enough.
First came the Crusade. That conglomerate of evil that was Chaos swarmed across the galaxy, cutting a bleeding wound through the body of the Imperium. The endless tides of men, twisted by the foul desires and evil within themselves tore apart their brothers, consumed men, soldiers, armies, and worlds. They were an unstoppable force. But they were their own undoing. As they cut apart the Imperium of Man their own power lessened, for the Ruinous Gods drew their strength from the warp, fueled by the darkness within men. The Crusade only weakened them. But that would have meant nothing, for their strength would have returned with the rise of Chaos worshipers, but the galaxy was far from calm.
Mere weeks after the blade of Chaos had blunted itself upon the Imperium did both fall under attack by a WAAGH of immense proportions, a force gathered together by the machinations of the Eldar for the sole purpose of driving might Chaos to its knees, in hopes of weakening the foul Slannesh. This mighty horde tore through the Chaos legions and Imperial defenses alike, scattering them as leaves in the wind. Of the four Imperial Segmentum two were in disarray. The blood spilled weakened the warp even more as the source of its power waned. Its greatest source of power for the first time significantly reduced. But it, Chaos, and the Imperium of Man would have all survived even this. But then they arrived.
On the other side of the vast Imperium a terrible enemy, implacable, unreasonable, and unstoppable. Tyranids. The Hive fleet infected worlds with its foul strength, consuming them whole and moving onward. With the reserve fleets stripped from the Segmentum for use against the forces of Chaos and the Orks they were unable to drive off the insectoid creatures that rampaged across their worlds. Soon the fleet had grown large enough that it threatened the last unchallenged Segmentum. With all four major Imperial forces in conflict men died by the trillions every day. Everyday the foul forces of the warp could draw upon less power, as the men of the Imperium were lost to them. Sadly this did nothing to stop the Tyranid advance. They moved across the galaxy in an unstoppable wave. All they faced fell. Through fate or by virtue of intelligence, they avoided Holy Terra and her mighty fleets, instead moving across the sides of the galaxy. In the face of the endless hordes of Tyranids there was but one solution for the Imperium. The massive forces arrayed against them could only be stopped in one way. Exterminatus. The utter annihilation of all life on a planet. And thus the Imperial forces pulled back to her core, using whatever means at their disposal to deny their own planets to the enemy. Cyclonic torpedoes, virus bombs, warp rifts. All these methods and more were used for the destruction of the planets of the Imperium. The losses experienced by the Imperium were immense. But it was still not enough. Though the Orks and Chaos were either destroyed by the destruction of the planets or by consumption into the Tyranid flee the powerful monsters drove deep into the Imperium, their bio ships enabling them to escape to space unlike the orks who needed their spores to land on a planet and their lack of dependance on the warp allowing them to ignore its weakening.
To stop this foe the Imperium used its full might, the remains of the fleets from four Segmentum and one fleet from the defense of Holy Terra. The Tyranids were ended in a cataclysmic battle that lead to the near destruction of Imperial forces. Their forces and strength reduced to a level that had not been experienced since the Emperor took charge of Holy Terra and mankind. Though they had destroyed their greatest enemies they were weakened to a level that could barely be comprehended. They worked furiously to recover. It was not enough.