(So, quick note. This has nothing to do with Nightmare to Come, despite sharing the same relative date. I find it rather hard to, while not compete, sort of exist in the same realm with arguably the most well-known 40k fic save TTS. Now that the distinction has been achieved, let's get started...)
It is the 51st Millennium, and the galaxy has been still.
The Imperium has been long dead after the cataclysm of the 42nd Millennium. Though few remain that saw the end with their own eyes, myths and legends state that the heroes returned, gods were born and slain, the ancient metallic horrors from beneath the surface rose, the Orks fought so hard and so constant they became nigh-extinct for ten thousand years, and the Aeldari finally died.
The Chaos Gods reduced to null, the Rift closed. The Traitor Legions found themselves cut off from their patrons, with many no longer blind to their heresies. The daemons of old became the stuff of bogymen and stories before bed.
The great tide of Tyranids were somehow quelled, this feat unknown save but tales of old men, which we will get to later.
But the close of the 42nd Millennium was not without loss. The Emperor, for one, finally allowed to rest after saving his people one last time. Terra, not destroyed but cut off without supplies or resources. Communication became rare, the Astropath Houses of the old Imperium reduced to but a dozen from the sheer casualties during the Final Wars of the age of the Imperium.
There is no unifying force in the Galaxy anymore. The Lost and the Damned have no gods, Humanity has no Emperor, the wild Tyranids no hivemind, the feral Orks no major Warbosses, and the Necrons have no Silent King. The Tau have no Ethereals, and the remaining Adeptus Astartes have no Primarch.
Across the galaxy there exists small pockets of civilization. A hundred thousand Imperiums, empires, dynasties, and kingdoms. A few worlds have the luck of having a strong military, others don't. While the galaxy is still, it is not calm. War is not gone, for border conflicts and resource wars occur commonly. Understanding is not here, for the cruel fates experienced in the Age of Strife, Age of the Imperium, and the Age After have created grudges that may never dissipate.
It is the 51st Millennium, and the galaxy is awakening...
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So the goal with this, alongside making a fun story, is also to create rules to go with the factions, units, and such to be introduced. They will be compatible with 8th and 9th, though moreso 8th because 9th isn't exactly complete in of itself. They will come with the post that explains their faction and their actions and wars.