Fall of the Eldar
The Fall of the Eldar is an event which has the dubious honour of setting the scene for the fucked up, grimdark galaxy of Warhammer 40,000 that we all know and love. Despite this, Games Workshop has released almost no lore from that time, mostly because the Eldar aren't Space Marines (and as such don't get ridiculous amounts of favoritism from GW's fanbase and Games Workshop themselves) and therefore won't sell quite as much as another entry in the Horus Heresy series or something. The remaining, actually forgivable, excuse is that Fall of the Eldar fluff may be seen as too deep and too risqué for the child fans Games Workshop wants to appeal to, as the nature of the few facts that are known about it are how the Eldar were so hedonistic their debauchery accidentally created the Chaos God of hedonism.
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, and it's hinted Dark Age humanity either never discovered them or purposefully kept its distance).
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 and sisters didn't believe them. And even more unfortunately, some of their brethren did believe it but actually wanted a god of pleasure to manifest because it's not like such a being would be evil, right? So they 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 spacetime, later known as the Eye of Terror, that 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. To make things even worse (for Eldar at least), the birth of Slaanesh consumed most of the energy that caused massive Warp storms all over the galaxy, and thus allowed humans to launch their massive xenocidal campaign during which many of the still young craftworlds were destroyed, and even more Maiden and Exodite worlds were colonized by humans.
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 came 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, their inhabitants doing everything they can to atone for the excesses of their ancestors. A few of the Webway-dwellers were rescued by and pledged themselves to Cegorach, becoming the Harlequins.
Even the Eldar's gods were affected. When Slaanesh was born, he/she/shklee went on an orgy (because Slaanesh goes on orgies, not sprees) of murder that devoured most of the gods of the Eldar pantheon. The only suvivors were Khaine (who could not be defeated by a god of something other than war, but was shattered into a million pieces after Khorne used his body as a blunt implement to beat Slaanesh into submission during the newborn god's rampage), Cegorach (who escaped into the Webway), and Isha (who was "rescued" by Nurgle from being raped and eaten by Slaanesh at his birth, but is now kept as Nurgle's prisoner guinea pig. Alternate theories suggest that Isha isn't actually held captive, but is staying with Nurgle willingly).
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 spacetime 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 any Eldar to tell them about it (b) most non-Harlequin Eldar are so arrogant that they think the Imperium, or humanity in general, are unworthy of being taught Eldar history (c) the Imperium arrogantly seems to think anything that wiped out the Eldar was because they are xenos, and that humans are the rightful inheritors of the galaxy. Given that the Chaos Gods specifically stated that their interest in humanity is because they are easily manipulated buffoons, it shows that humanity in 40k has at least one thing in common with the Eldar: high levels of arrogance.