OK. This has evolved over several years, and a few things have to be understood (in my universe anyway). The Milky Way is NOT the 40k galaxy in my game, it was oing to be
before they made more discoveries about it. Still might be, I'd just have to make it a little larger and not as degraded. Anyway, it could easily work as any of the other close neighbors (
) I chose Canis Major, specifically, because it was currently colliding with our galaxy.
Also, it was the 41st Century during the 2nd age of Earth in mine as well.
Well, here goes...
After Abbadon's last crusade chaos went quiet for a while, too quiet. All signs pointed to something bad happening, Increased Eldar activity, the malfunctioning of the Golden Throne, etc.). Eldrad Ulthuan's apprentices (who still held his still active way stones) started having visions so powerful that they felt could NOT be ignored, and felt so intently they sought other craftworlds to help, asking that differences be put aside. These same visions were had by powerful Earth psychers and Librarians. All the visions let to a singular point in space, a small gathering of stars with 1 being of prime importance, Sol.
Abbadon has found a way to navigate past the galactic edge into another galaxy. The point of navigation is only safe/clear through a narrow corridor a few light years wide. (Sol, Alpha Centauri, etc). This would make capturing and holding those stars and planets key to controlling an entire other galaxy, ripe for corruption. (Think Cadia and the Eye).
Scan forward to a MASSIVE battle in and around the Sol system with Chaos against Eldar, Space Marines, Imperial Guard, and Tau. ( I even have a list of the SM Chapters, Craftworlds involved, etc).
Insert the first Scourge of Earth. The battle was, at best, a stale mate. They defeated Chaos (mostly), but the cost was too high. The crash of 3 craftworlds, the complete loss of at least 4 entire chapters (and the crippling of several others), innumerable human lives lost, and the loss of the Tau Fire Clan and Etherals (Nurgle plague, hehehe).
his battle has also caused the local Warp/Astral to become unstable, making their way home unnavigable, and changing the way the Warp affects localized space. The ripples caused here will be felt for thousands of years. (mana cycles).
The years to follow helped shape generations to come. The remaining Tau fled farther into this new galaxy in fear of what will happen to them, as did the remaining SM chapters that could (in search of new stock and a new home). The Eldar and Humans had to survive on a planet already inhabited....by Dragons (descendants/relations of the Old Slaan/Old Ones). Eventually, an equilibrium was found and everyone could try to understand what has happened and figure out was to come.
With the mana cycling down
ALLOT (thousands of years) sooner than it should have and the still influencing effects of chaos (and the lack of soul stones, or the ability to create a world circuit) the Eldar were getting nervous and the Dragons concerned having to go into hibernation with all these....lesser beings scuttling about.
A young and very ambitious dragon (Alamais), sought out the leaders of the Eldar with a proposal that could benefit all involved. He has devised a ritual/process that could mask/hide or "piggy back" Eldar DNA within Human DNA (keeping their soul linked to a human one and away from Slaanesh) until the time when the Dragons would re-awaken and a more...palatable solution could be figured out. Depression and desperation were the only reasons they agreed...and the Elves were born.
Over the course of the eons the blood got mingled and the lines of "race" blurred. Abhumans were intermingled with human, human with elven, etc. Then the Awakening of the 4th world. New races developed (Obsidimen, Tskrang, etc) though stable mutations of the mixed DNA and the Name givers barely resembled their forefathers except for the Humans, Dwarf (Squat), Elves (Eldar), Troll (Ogryn), and the most unusual of all Orc (a blending and mutation of geneseed).
Through the manipulations of a faithful few (Alachia to name one) Alamais' betrail became apparent. He had no intention on helping the Eldar back to their original form. Why would he, when he has successfully created a servitor race?........skip forwar...The Scourge (Chaos returns, Daemons mostly, through the Warp/Astral).......flash forward again...
The 6th World. Some dragons trying to position themselves to ensure the Eldar never resurface, some trying to see they do. Ares finding mummified several remains on Mars and starts trying to clone them for study, eventually leading the their discovery of the geneseed and the attempt to re-constitute it (hence Project Spartan)...etc..
These are just the highlights. I have
ALLOT more detailed out, but...this should be enough I hope.
I also have 1 other Sci-Fi background thrown into my mix concerning Old Ones/Eldar way gates, but that is a scale of a different color.