Fall of the Eldar
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The Fall of the Eldar has the dubious honour of setting up the scene for the fucked up galaxy we all know and love. Despite this, Games Workshop has released almost no lore from that time. The reason for that is due to their ridiculous hard-on for Space Marines they're too obsessed with writing fluff for the Horus Heresy, while sadly retconning out any of the good fluff they'd already written about it.
History
The Fall was set in motion by the War in Heaven between the makers of the Eldar, the Old Ones, and the Necrons and their C'tan overlords. The war fucked everyone over and everyone that didn't go to sleep got nom-nomed by psychic space terrors.
In the aftermath, the Eldar found the galaxy was theirs for the taking (the Necrons having spent themselves and gone into hibernation) and so became its rulers. Through a combination of their advanced technology, the remnants of the Old Ones' Webway, domination over their psychic powers, and long life spans, they had little to no opposition (any Ork WAAAGH too stupid to start up most likely got dropped down a black hole or something).
After many millennia of galactic dominance, the Eldar started to lose interest in menial tasks and concentrated purely on pleasurable acts. Given the depths of their emotions and sensations, these acts soon descended into depravity that would make human snuff films look like Saturday morning cartoons. And then, there were some Eldar who set up private realms in the Webway so that they could commit depraved acts that went too far even for most Eldar. The Empire's collective debauchery, amplified by their psychic prowess, started churning the Warp itself, making Warp travel nearly impossible.
Some Eldar became uncomfortable with the direction that their society was headed, and settled worlds on the fringe of the galaxy, away from the advanced technologies of the Eldar Empire, and returned to a simpler lifestyle. These were known as Exodites. Others dwelled on massive Craftworlds that traveled the whole Empire, and from their outsider's perspective, could see where things were going. Unfortunately, most of their brothers didn't believe them, and kept right on fucking away, until they went and made a new Chaos God -- Slaanesh.
The birth scream of the great pervert tore a great orifice in space-time that was later known as the Eye of Terror and consumed most of the Eldar race instantly, even the ones who hadn't wanted to join in with the whole decadence thing but were too close to the epicenter. The only ones who escaped were the Craftworld and Exodite Eldar who were near the rim of the galaxy, and the Eldar who were living in the Webway. The consumed worlds became known as Crone Worlds, a particular variety of Daemon World.
Aftermath
Thus the Eldar are now a fractured people. The ones who lived in the Webway found their souls slowly draining away over time, and discovered that they could drink the pain of other sentient beings to replenish themselves. They re-organized themselves to gain victims more efficiently, joining their private realms into what became Commorragh, and coming to be called the Dark Eldar. The Exodites continue to survive on the rim of the galaxy, and the Craftworlds still intact drift through space. A few of the Webway-dwellers were rescued by and pledged themselves to Cegorach, becoming the Harlequins.
Even the Eldar's gods were affected -- of their pantheon, the only suvivors are Khaine (who could not be defeated by a god of something other than war, but was shattered), Cegorach (who escaped into the Webway), and Isha (who was rescued by Nurgle).
The birth of Slaanesh had the positive side-effect of providing an outlet for the pent-up Warp energy that had impeded Warp travel for so long (well, the outlet was "tearing a great orifice in space-time and obliterating the Eldar Empire", but at least it was cleared up once the event was over). Warp travel became possible again, which allowed the Emperor of Mankind to launch his fleets and begin the Great Crusade, and without the mighty Eldar Empire to contend with, his new Imperium of Man could grow virtually uncontested.
Hilariously, the Imperium seems vaguely aware the Eldar had a big old space empire at one point but ignores this fact because (a) the Imperium won't keep their hands off their guns long enough for an Eldar to tell them about it (although the Eldar are so arrogant themselves they might think relaying the information is beneath them...) and (b) the Imperium arrogantly seem to think anything that wiped out the Eldar was because because they are Xenos, and humans with their handsome faces and clean skins are just sooooooooooo much better. Given that the Chaos Gods specifically stated that their interest in humanity is because mankind is the only species to ever claw its way back to the top after being fucked-over by Chaos (where-as the Eldar sure-as-shit did not) and then survived ten thousand years of crap that is easily destroying Craftworlds at least proves that humans are better at surviving, overcoming, and just plain winning than the Eldar.