How would portray this third group of Eldar?
I am thinking of using what is in RT for the Eldar already and just using the Psyker Archetype as a basis. More than likely keep them Slaanesh aligned.
How would you have them look? More bare chested with tattoos and other psychic defenses or to the extensively baroque suits of armor and brightly colored so they can experience a full array of sensation. I see their mutations being far more subtle to enhance that natural grace and allure of the other Eldar. Only the most dedicated leave the Crone Worlds to enact the will of Slaanesh to bring back their cousins to the embrace of the Dark Mother. A corrupt Eldar ship with the most terrible technologies of the Eldar at their height with the reality bending techno-sorceries of the Warp. How would you display their ships?
I also see them being allies of the a Heretic Coven since they serve the Dark Gods, even more so if that Heretic is aligned to Slaanesh.
One more thing, Slaanesh specific powers for the Crone World Eldar?