Had a thought while replying to another thread: There is the nigh unspeakable Path of the Damnation which falls under the Outcast.... http://wh40k.lexicanum.com/wiki/Eldar_Path So, perhaps, there is a demonic Sorcerer Eldar out there, unchecked and unbridled in power. That is a terrifying prospect considering just how powerful that pysker would be... Boobs. Which would be a pretty awesome PvE experience if you ask me... maybe even a "secret reason" the Eldar are on Arkhona. What do you think of a deranged Eldar that's gone rogue pysker? That character, as we all know Eldar to be, could be one of the most powerful pyskers in the galaxy....
Well, what would set them apart from a successful rogue pysker within the Imperium? A creature with an affinity for psychic powers on top of thousands of years of training and preparation (purposeful or not) would probably be one of the greatest champions of Chaos to ever live.
The fact that Slaanesh already has a direct claim on his/her soul. There's a reason why Dark Eldar prohibit the use of their latent abilities.
An yet, it would seem that if an Eldar could gain enough favor from another god, they may find salvation - as Demonhood generally makes one immune to things as meaningless as death.
Not saying it is entirely impossible (because 40k excels in the ridiculously awesome) but, an Eldar would have to be completely arrogant, uncaring of being tortured for eternity, and extremely powerful to pull it off. It is very unlikely that's all.
I dont know if its possible but it sounds like it could make for an interesting idea for a character. Maybe not for EC though but I suppose its plausible that one upon a time Tzeentch said screw you to Slannesh and made a deal with a deranged Eldar. Or maybe they started grew up on an exodite world that was attacked by orks or something and never new of the Chaos gods or something.
Not sure if anyone's seen Dexter, but a Khornate Eldar - especially a psychopath from Commongrah also doesn't seem too far fetched. A self righteous Eldar who thrives on spilling the blood of those he deems unworthy (describes almost every Eldar...) is exactly the kind of arrogant murderer I'd expect to see. That, and Khorne is directly opposed to Slaanesh, while also being quite close to Khaine in perspectives. I'd definitely see Khorne as a possible bastion for Eldar souls he'd like to keep Slaanesh away from. And again... the sheer power these warriors would be capable of... *shudders*.