Warhammer 50k: Clawmarks on the Throne

"It has become a firm belief of mine that the final battle in this wretched universe will be waged not with the holy melta. And neither with the sacred plasma rifle. No, not even the humble lasgun shall be present. No... the final battle shall be fought between brutish muscle of the greenskin and chitinous claw of the Devourer..."
- notes extracted from the memory-drives of rogue Magos Biologis Xathur shortly after his discontinuation
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It is the 51st Millenium, and Holy Terra has fallen.
How did it all come to this, one might ask? Some will point at a devastating blow dealt to the Imperium by the Ruinous Powers through their new Warmaster. Others might blame the mind-witchery of the unholy xeno 'Drudgelords' from the rising Tau Singularium. And then there are those that, albeit in hushed tones, attribute this disaster to the internal failings of the Imperium itself.
Regardless of which series of unfortunate events one might look to, they all culminate in a relentless onslaught wrought by several Orkish WAAAAGHS! and Tyranid hive-fleets rampaging through the Sector Solar, eventually overwhelming the Imperial defenders. Now, the sacred world that used to draw billions of human pilgrims to its vast space-ports has been reduced to a battleground between the two savage xeno-species.
But alas, all is not lost! The Astronomican, although weaker in strength, is still operational. And paradoxically enough, faith in the Imperial cult has never been stronger amongst humanity, with visions and sightings of the God-Emperor becoming increasingly common.
So as darkness draws nearer from all sides and countless heroes rise and fall, one thing remains certain; the grim darkness of the far future isn't going away any time soon.