Pre-40K Retcon (AU)

 -PreHumanity-

The Ctan were created by the Old-Ones, thinking machines, but ones gone rouge.

The Hrud were the first allies of the Ctan, and were defeated.

The Ctan were "jailed" in a star-fortress (type of blackstone fortress).

Necrontyr expand, whilst not defaultly genocidal, they are enslavers, they enslave the demilitarized Hrud and brush the borders of Old-One territory.

Necrontyr were a relatively young race and were impatient for technology from the Old-Ones, the Hrud told them of the Star-Forts.

The Necrontyr scoured the forbidden sectors of space until they found the Star-Fortresses, the Necrontyr interfaced with the Ctan for their information, activating defencive protocols, and thus starting a "2nd" "war in heaven".

The Old-Ones were brutally ruthless and inflicted a viral cancer and infertility on the Necrontyr, and sent a bio-warrior race known as the Krorks to wipe them out. The Necrontyr were losing.

Arguably they put the Necrontyr in a corner where they had to accept Ctan's offer to make them into the Necrons, and in order to survive do onto the Krorks & Old-Ones the like of what had been done to them.

Within a thousand years across the galaxy every Necrontyr had been turned Necron, the remainder became extinct, they became a wholly warrior race capable of going toe to toe with the Krork, and onto the Krork a degenerating plague spread rendering them closer to the Orks we know with each generation.

The Hrud and a number of other slave-species were also converted.

The Necrontyr were in spite of this now slaves, and the Old-Ones had but one move left to them. The Old-Ones freed the minds of the Necrons. (Some go mad, as their form can't process it, they become "flayers") The Necrons turn on the Ctan, leaving themselves vulnerable to the Old-Ones. The Hrud use the the opportunity to flee into hiding.

The Necrons make a pact to enter hibernation (or leave the galaxy) using the Ctan shards as assurance.

-later-

The Enslaver plague wipes out most of the Old-Ones. The Old-Ones segregate, become cut-off, unwise, relient on client species. Such as Zoat and later the human's that will be known as Eldar.

The Tyranids are alerted to our galaxy when a Zoats colony-ship discovers a space-hulk of genestealers from before even the Old-Ones time.

The Krork are slipping into Orks, their old-culture is all that keeps them working for the Old-Ones, but each successive Warlord is becoming too stupid and bold to follow mundane directives.

-elsewhere-

Humanity spread from earth, becoming
-Humanity-Proper: Us.
-Men of Iron: AIs that ruled over and assisted Humanity-Proper
-Eldar: Absconded AI rule, gene-edited, became vassals to the Old-Ones
-Voltan: Gene-edited human workforce of Humanity-Proper who many AI's came to favor over humanity-proper.
-Uplifted: Jokaero & other -Splicers: The "Beastmen" adapted to their environments by splicing with animals, but like many would suffer for it during the crusades and forever after.

Humanity-Proper implodes in an AI War between Men of Iron Humanists (AI's of Voltan) and Anti-Humanists (Robots-Only).

Humanity wins after the silicone plague is introduced by Humanists.

Humanists implement a vote for AIs place in human society, it is massive and expansive 80% of all humans vote, many humans don't even believe the AI's will adhere to the results, perhaps in itself exacerbating a landslide decision for AI to exit the stage.

To the horror of many the AIs adhere to the result and exit the stage in human affairs focusing souly on their Voltan children who themselves voted vice-versa.

The Primarchs were left behind by the AI's to guide humanity, more human than AI, but with the ability to process huge quantities of information. The Silicone plague still roams large and stunts efforts by individuals to recreate AI systems.

Within thousands of years after the exit of AI, humanity collapses, it is unsuited to governing, it was too soft and in many cases to liberal and corruptible to effectively administrate on a planetary scale, nor interplanetary, nor intergalactic scale.

The Primarchs are mostly ignored, shunned and pushed to the fringes of civilisation, often killed as Goliaths imposed on David.

Many worlds fall apart into factions, that between themselves ruin their worlds fighting and extracting resources.

Earth is one-such world.

On earth the "Thunder-Kin" exist on a secluded continent of ruined-Earth, gene-edited soldiers of the-before, they outcompeted humanity-proper, but were not technologically savvy due to their mastery over their environment, and have long lost their edge to humans-proper.

A primarch who will become known as "The Emperor" has wondered the galaxy throughout the collapse, he was part of an expedition to earth proper fleeing from war.

Only they arrive to see earth little better than a wasteland and are shot out of orbit by frightened warlords on Earth fearing the Martian servitor harvesters.

-elsewhere-

The Eldar have thrived in the retreat of humanity, they are more adapted than humans to self-govern, and they are loosely indoctrinated by the Old-Ones to assist in their great works.

The Zoats have long since exposed the threat to the Tyranids to the Old-Ones, the Eldar have been instrumental in building up the defences for this war to come against the Tyranids.

However, scholars of the Eldar begin to dive deeper and deeper into 4th dimensional crafts, somehow "crafting" the "Eldar Gods", only in their pursuit for a war-machine to harness their collective energy they manifeste of Slaanesh.

The Craftworld eldar had warned against this arrogance and fled in advance, though many would be too late.

Slaanesh consumes the old-ones, many of the Eldar-Gods, and much of Eldar society.

Many Psi-enabled individual galaxy-wide are alerted by the Old-Ones of an incoming cataclysm of both the Tyranids from the great-beyond and this new wound sure to fester within.

Regardless of the fall, of the Eldar and Old-Ones, later wars are being fought, by Old-One constructs (blackstone fortresses + drones) and few remaining Krork and sworn Eldar, to hold back the Nids.

Zoats still scramble along the outer rim of the Galaxy trying to create alliances, partly to atone. (The Voltan are in painfully slow talks with their support for this war)

The Rak'gol who lived in the furthest reaches, are in retreat from the Tyranids.

-elsewhere-

The Emperor survived his crash onto earth's surface and lived for many human generations as a simple herder, sage, and warrior. He like so many others had the vision of the cataclysm.

He scours earth and finds great supplies of weapons and the gene-edited warrior peoples of unexplored lands. Long frustrated "The Emperor" decides he must lead humanity to stop the cataclysm.

The Thunder-Warriors (said gene-edited warrior peoples) are promised their own planets and kingdoms in return for service, they are thus armed and supported by the humans-proper that swiftly unite behind this unstoppable force known as "The Emperor".

This new Imperium spreads through the Solar system and then a number of other systems.

The Thunder-Warriors become discontent, they not simple tools, they want to stop war and settle their own worlds as promised, The Emperor sees this will be a problem, and that in the long-term these peoples will divorce themselves from humanity-proper.

He thus recreates the relics used to create the Thunder-Warriors so long ago, only this mutation cannot be passed down, the mutation only affects boys, it also renders them infertile. Forever more this new type of super-soldier will be tied to the fate of humanity-proper.

For a time with both Marines and Thunder-Warriors work side by side

The Custodes signal the end of the usefulness of the Thunder-Kin-Proper. Like the marines they are infertile, but seem not limited to one sex, they are spliced with a slither of the Emperor's own encoded DNA. Unknown to neigh-everyone it is vat-grown Thunder-Kin offspring are being used in the place of human offspring.

The turn on the Thunder-Kin, their gifted system to rule is taken back, they are culled or reduced to slaves, and eventually rendered the vat-grown Ogryn who are little more than ape-like oxen.

---

The Emperor is not uniquely better than any other Primarch (other imparted wisdom from old-ones), as his crusades expanded he laid down a mythos of finding his "sons", in reality killing Primarchs that would not submit. He would often bring planets to kneel, rather than engage in fanciful competitions to prove his worth, but happily propagated these tall-tales in the imperial propaganda machine.

(The Propaganda machine in charge of propagating these arthurian-esk legends would later become the ecclesiarchy.)

Horus and his alliance were not corrupted by chaos until they scattered into the warp, they were stone sober officers disillusioned with the Imperial expansionism the Emperor sought, and subjugation of fellow primarchs, for some nebulous prophecy of a Cataclysm.

When Horus came to Terra it was under truce, with Sanguinius (the most well liked of the "loyalists"/"unionists"/"rationalists") agreeing to discuss terms, Horus wanted separation, into confederations, Sanguinius tried to convince him otherwise.

Imperial Assassins swarmed this meeting seeking the kill Horus, Horus's guard prevailed, and Sanguinius was arrested (though he knew nothing).

Magnus tried to warn Sanguinius of this plot, but his message was intercepted.

The Emperor shot most of the enemy fleet out of orbit, the crashing ships killing millions, many of the "invaders" were simply survivors marooned on earth, in most cases the swarms of attacks were fleeing civilians and later those brought to looting, only a fraction of the "fighting" was against an organised opponent.

The flagship on which Horus and Sanguinius were stationed was the first to be eliminated.

The Anti-Imperialists fled with parting shots that killed billions on earth, and eliminating a fair portion of the Imperialist fleet considering.

As Imperialists converged the only "safe-place" left was "The Eye of Terror", a place that warped reality and the minds of all that harboured there.

-elsewhere-

Rak'gol were in a feral retreat from the Tyranids, masses of their sub-lightspeed colony-ships are only just arriving in many systems, unable to fight toe to toe with the imperium they have had to adapt, becoming a race of corsairs.

Though very capable the Voltan are painfully slow to organize against the Tyranids. Some stick by sworn oaths (codes they must abide by else activate their "kill-switches") to leave the galaxy to its own devices.

The Tau have mobilized fighting-expeditions, but their supply lines are stretched, and the Ethereals would prefer to concentrate their efforts locally rather than indirectly aid the imperium. These missions are perceived as convenient ways to get rid of political dissidents, whilst building goodwill with the Eldar, Voltan and other xenos.

Rumors exist that some Necrons rise to heed a pact made with the Old-Ones prior to stasis, but as "inorganics" they are dragging their feet, many more are outright not honoring a pact made in duress.

The Imperium's Ordos Xenos, and indirectly Deathwatch, have made pacts with xenos to defend against Tyranid incursions, the Tau owe their existence to it.

-40K- (Fluff retcon)

Proven Loyalist Chapters permitted 10,000 marines-proper and have further allowances (Knights-Errant, World-Guard, Neophytes (Without Black-Carapace), Aspirants, etc).
Newer Foundling chapters have to jump through hoops to be approved.
(the Minotaurs gained this in the Badab War, Lamenters lost it there)

(These limits do not apply to Deathwatch as their manpower is borrowed from each Chapter's Marines-Errant, though in theory their Black-Shields are a part of these restrictions.)

There are penalties for (proto-name) Knights-Afoul, a point of contention for the Dark-Angels, in that Chapter's cannot delist personnel that go rouge, they eat up space until confirmed dead.
Should that list exceed active numbers the Chapter is meant to be disbanded.
Dark-Angels have slipped into this multiple times, though they try to whitewash the fact.

There are around 4 million marines-proper galactically, it was 10 million prior to the Tyranids.
The "primaris-gene" boosts Neophyte survival odds, a little more robust and less prone to malformation. (i.e not as significant)