(READ AUXILIARY CHAPTER, THERE IS IMPORTANT INFORMATION THERE.)
Around the galaxy, warp sensitive individuals feel a distinct shift in reality, sparking confusion throughout many of the races, as the chaos gods, sitting within their respective realms look out at the strange occurrence, as the core of the galaxy opens for the first time in eons, long locked to the outside, strange anomalies prevented warp travel to the core, and anyone dedicated enough to try to enter through relativistic travel, would find themselves traveling for eons, seeming to make less and less progress as time passed, as if space itself began to grow as they went, nothing ever went in or out of the core. The only difference from a black hole being that it would reject any attempts at entry.
But suddenly things changed, the warp shifted, and new paths formed from the chaos of the dimension, despite realspace still showing the anomalous ability to reject all entry, turbulent rivers formed in the rift, welcoming the most foolhardy or crazed of travelers to explore the unknown, something this world had no lack of, the numerous factions of this universe looked on with greed, eagerly beginning to send party after party of explorers into the rifts, morals and sanity had little place within this universe, as factions would consistently partake in planet wide genocides, with most willing to sacrifice trillions of their own for nothing more than a single foothold, history speaking of times when the imperium sent billions of guardsmen to their deaths, just to sack a single city, on a strategically unimportant moon.
The scouting parties that survived the warp, a dimension of pure insanity and chaos, capable of devouring entire armies whole at even the slightest mishap, found themselves in derelict systems , the ancient remains of long destroyed megastructures and debris fields of ancient dreadnaughts, planet craft, and similarly imposing weaponry speaking of a violent end, a common sight in the universe of 40k.
 But they spent little time exploring, near instantly entering combat, as armies began to pour in behind them, space quickly filled with the lights of destruction, beams of energy, esoteric or materialistic, joining each other in a cacophony of violence that would make Khorne proud, as the long lifeless planets were once again stood upon by living creatures after Myr's of silence, only to quickly be relight with the fires of combat, ancient ruins destroyed as the planets they sat upon were repeatedly glassed as they the factions that fought for them lost ground.
The core systems quickly becoming nothing more than another battlefield like the many others found across this crazy universe, but things stood to change in one small part of the galactic battlefield, within the central system, the absolute core of this galaxy, sat a relatively diminutive structure sat, strangely orbited by celestial bodies with many times more mass, the insanity that was the 40k universe, where anomalies are so common they degrade the very definition of the word, such an oddity was simply ignored, the small planet, just larger than earth's moon, was ignored in favor of the light year diameter megastructures populating the surrounding space, the ruins a testament to whatever civilisation built them.
Not that the planet was ignored, not when factions would sacrifice billions for mere meters of land. But their main armies were simply focused elsewhere, ignorant to the fact that they would live to regret their actions, not that it mattered in the grand scheme of things.
Focusing on the planetoid the space was filled with the wrecks of ships, both ancient and the wrecks of the current fighters. As the surface was turned to a hellscape, weapons of mass destruction deployed with as little consideration as grenades, in one corner of the globe a imperium detachment fought tooth and nail, dying in droves as crazed green orcs ran at their lines with reckless abandon, their race caring for nothing more than battle, their entire civilisation based off of violence, in another corner tau forces, still new to the galactic scene, fought waves of eldar in a much more organized but similarly brutal fashion, all as each and every factions is pounded on all sides by the corrupted forces of chaos, the corruption of Nurgle festering in the mountain high piles of decaying bodies, as Khorne reveled in the brutality of war.
The most important point of the war taking place, quite contradictorily, on the smallest front.
(john pov)
My name is john, a imperium guardsman, and part of the core system expeditionary force, I joined the guardsmen just recently, hearing about the new crusade traveling to the newly accessible core systems, I was eager to prove my worth to the god emperor, joining the scout corps, I was lucky enough to survive the travel through the warp, as I thanked the god emperor for giving me this chance to prove myself, I was disappointed when I stared in awe at the newly arrived system, gargantuan structures, larger than anything I had ever heard about, lay floating lifelessly in the void of space, I briefly felt dread at what could have possibly killed such a grand civilisation, but I quickly hid my heretical thoughts, unwilling to let my commissar notice my dread, lest I be executed for cowardice before receiving a chance at glory in battle, but I still shiver as derelict ships, larger than even the battleship I crewed, despite being a mere fragment of what they must have been in their prime, but my thoughts were interrupted by loud sirens.
My ears bled as I rushed to my commissar, eager to learn the cause of the alarms, the multi kilometer long ship I crewed, a veritable city on its own, far to large for information to travel quickly, I was informed that we had been rammed by a detestable orc raiding ship, at the opposite end of the ship from myself, I ran to join the battle, eager to spill orcish blood in the god emperor's divine name, but it was not to be so, as the ship began to crash upon one of the smaller planetoids of the system, I ran to my units landing craft, as we all left the stricken ship, watching it crash in a blazing glory, I briefly prayed for the souls of my fellow guardsmen, jealous of their brave end, and subsequent joining of the god emperor.
But I would soon be given my own chance, the dropship landing, me and my unit poured out, quickly digging trenches and preparing for war, peace lasted but moments as our lines were beset upon by orcs. Hours of battle passed, and I thanked the god emperor that I survived the first few assaults, statistics showing the average guardsman lasted less than an hour in combat.
The commissar rallied the men, and I prepared to charge against the enemy lines, our target a stomper in the distance, we had the assistance of the space marines to bolster our moral, A sharp whistle filled my ears, as me and my comrades vaulted the trenches, and ran across no mans land, I watches as my comrades were gunned down on mass, artillery and explosions landing around us, as I fired my lasgun ahead, uncaring of accuracy, when suddenly the ground gave out beneath me, and I feel down a deep crevice, previously hidden beneath the soil, I noticed, surprised, that below the rock and soil that we had fought on, there was a complex metal shell, suspended above a moon sized metallic ball, complex geometric patterns covering its entire surface, I accepted my imminent death as I took in the grand sight, I landed hard on the surface, having somehow decelerated massively just before crashing into the ground, I lose consciousness upon the impact.
(an, how do you like the first chapter? I think I did a good job, I tried really hard to embody the insanity and violence of 40k, and the lack of care for sentient life as a whole, so I hope I did a good job, I wrote this at midnight, so my mind wasn't exactly working at full capacity when I did it, but I think it turned out well nonetheless.)