It's less about evolution, apparently, and more about mindset. As the Eldar are an inherently psyhic race. The Dark Eldar have largely repressed this part of themselves, since dabbling with pyshic powers and witchery attracts daemons, which is a big no in the Dark City. And yet they still retain some connection, because they literally feed on the pain of their victims (and other extremes of sensation, but mostly pain). But according to the Forge World book featuring the Eldar, where most of our current information on Corsairs comes from, they have been known to join their ranks by basically falling down the same road that the forerunners of the original Dark Eldar did so many thousands of years ago. Their time of freedom from the structured life in the Craftworld doesn't stop, and instead of eventually returning as most do, they continue on down the road of excess.

This would seem to be the logical turning point. Where that excess and their own personal gratification becomes all consuming, until any difference between them and the original Dark Eldar becomes academic. And once they've gone that far, any sort of redemption and return to the Craftworld becomes excessively difficult, asuming it's even desired. This also seems to be partially due to an effect of She Who Thirsts upon the entire Eldar Race as well. As the Craftworld and Exodites both deny themselves in some way, in an effort to fight her pull on their very souls. The former through their almost ascetic caste society, and the latter through constant toil in tribal and early neolithic societies. The Dark Eldar however have given in to the extremes of emotion that Eldar can experience, and so now they seek to fight against Slaanesh pretty much by feeding it, constantly fighting to replenish themselves, and going to increasingly extreme lengths to do so.

So if we look at it as mindset rather than genetics, we can see where things can all go horribly wrong. Since despite all their arrogance and advanced technology, the Dark Eldar are practically in the most tenuous position of all the races in 40K, with them not only preying on all others to survive as a species, but each and every one of them in their own personal fight for survival for every moment of their gloriously miserable lives. Because they don't have the same reassurance that upon death their soul will join the Infinity Circuit or World Spirit, but will instead be devoured by a horrific Daemon-God unless they're one of the lucky few who can afford ressurection, something only available to the elite of Commorragh, which comprises even the lowest Kabalite warrior (who are at least equivalent to minor nobility). So with the stakes of survival being so high, they can ill afford to ignore any chance, no matter how truly unpleasant to fight for it.

At best, the 'switch' would take a generation or so. If the circumstances of birth are that important. But while this is all as much conjecture as it is book fluff, I have to say I can't remember seeing it said specifically anywhere that the Dark Eldar and Eldar are literally different species at this point, even within the new Dark Eldar Codex. And that the differences are more along philisophical and cultural lines.

Okay, we seem to have been veering away from the actual topic of this thread.

I'd like to be more precise on my idea of the adventure supplement I'd like to see. The Koronus Expanse is uncharted territory for the most part, right? How about a campaign book about an entire crusade to claim vast swathes of the Expanse for the Emperor, with the players' Explorers as part of a whole bunch of different factions and organisations (Navy, Ecclesiarchy, Guard, Inquisition, Assassins, representatives of other Rogue Trader Dynasties, etc.), all united for a massive campaign of conquest.

In the first stages the objectives would revolve around amassing the forces, forging alliances, drawing support and acquiring all the hardware necessary: weapons, soldiers, equipment, void ships, landing craft, and so on, possibly trying to reach an agreement with a Space Marine Chapter operating nearby (looking for lost chapter relics?) or even some xenos (Kroot, Stryxis, Eldar?).

With the proper conquest campaign on its way, crusade-internal politics and conflicts could be very interesting (see also: Gaunt!), with each of all the different factions trying to get the most out of the campaign's gains and plotting against each other, trying to settle old scores or igniting new rivalries. Perhaps even the Ruinous Powers have started to corrupt the crusade's heart, increasing the internal strain.

Finally, the PCs need to save the day - and the crusade - by keeping the forces together and vanquishing all the traitors (or just incompetent drags), with the possibility of making new friends as well as new powerful rivals.

I think such a scenario would be a huge and fun sandbox allowing for countless possibilities for space fights, planetstrikes, plotting and scheming and backstabbing, shoot-outs, dungeaon crawling, looting and pillaging, in short all those things that're fun about Rogue Trader! One could integrate ties (or grudges) with Space Marines and xenos and it would also be the perfect possibility to - finally - add an Eldar PC career path in order to allow players to play a Corsair or the like who has decided to manipulate the crusade towards the goals of his corsair force/the craftworld he's in league with.

Such a book would make my wildest dreams come true…

Cryhavok said:

The difference between the Eldar and Dark Eldar are in no way evolutionary. Asdrubael Vect, the de facto leader of the Dark Eldar, supposedly even witnessed The Fall.

The Dark Eldar were simply the Eldar that did not change their way after The Fall. All of the Eldar - Craftworld, Corsair, Exodite and Dark Eldar - are the same species.

Some people seem to think that the "basic" Eldar, Craftworld Eldar, regularly simply called "Eldar", are what the Eldar were before The Fall, but arguably, they are the ones that have deviated the most from the Pre-Fall culture of the Eldar as a species.

I shall answer both threads on this topic!

My understanding is that there are three types of Eldar: There are Eldar, who are the members that heeded the warnings heralding the Fall of the Edar and fled, escaping mostly unharmed in their Craftworlds. There are the Dark Eldar, who fled once the Fall happened and they were "touched" by the birth of Slaanesh, which is why they have to sacrifice people to save themselves. There are also Exodites who are Eldar that grew disgusted with their race's ways long before the fall, and fled to the edges of the Galaxy and live much simpler lives although they still possess some technology. 

There are two types of books that I'd like. The first is an "Empire Creator" to provide guidelines on how to lost civilizations of Man or Xenos empires in the heart of the Expanse. Essentially a book with the breadth of Into the Storm, so that we get a wide range of information that can be peppered into the campaign as we see fit. The Xenos theme is just so we get an idea of how we can formally build our own Yu'Vath, egarian or Slaugth-type empires who have a lot of backstory in the RT setting but it's a little vague on how to build your own ships for.

The other would be a high-level campaign that could serve as a capstone to a party's adventures in the Expanse. It's really easy to do a lot of things in RT, but having a suitably long-term scope that would be a great note to go out on.

Galaxy Guidebook: Go outside the typical rpg setting and explore famous 40k locations like Terra, Mars, etc.

I'd like to see Ships and Stations expanded on further.

I'd also like them to look at the following:

Chaos ships still in use by the Imperium, such as those in the Reserve Fleets of Battlefleet Obscurus  (You know, the segmentum that the setting takes place in?).  Since, for no particular reason, the stats that would allow us to reverse engineer them from Battlefleet Koronus are missing.

Canon Imperial hulls (not including battleships) that have not yet been introduced such as:

Charybdis class grand cruiser  (IA 10)

Siluria Class Light Cruiser  (BFGM, BFG FAQ 2010)

Cardinal Class Heavy Cruiser (BFGM, IA 10)

Viper class destroyer (BFGM, BFG FAQ 2010)

Praetor class destoyer (mentioned in several sources but no stats ever published).

Long Serpent class Battlecruiser.