Here's the background I'm working on for my Eldar army. Please feel free to let me know what you think and how you think it could be fleshed out.
The Craftworld Sinn Achai was one of the last to flee the Eldar Empire that actually survived the birth of She Who Thirsts. As a result, its occupants were some of the least puritanical of the survivors. Due to their late departure, they were ensconced in the privilege and luxury that was so common on the late Eldar Empire. Once their initial supplies ran out, the occupants of Sinn Achai quickly realized that they lacked the skills to survive without the infrastructure to which they were accustomed.
Initially, the Sinn Achai Eldar reached out to other Craftworlds for help. Unfortunately, their late departure meant that the more puritanical Craftworlds believed them to be tainted by the hedonism that had caused the Fall. The lack of skilled individuals on Sinn Achai only supported this assertion.
Desperate and starving, the Eldar of Sinn Achai stumbled upon the Exodites of Jacurutu. While most Exodites colonized lush Maiden Worlds, the Exodites of Jacurutu were more puritanical than most. They had chosen a brutal desert world to drive out the hedonism they felt was inherent in the Eldar mind. The struggle for survival had turned them into a lean and hardy people. In the Sinn Achai, they saw kindred who were desperately in need of the lessons of the deserts. They offered to teach their Craftworld cousins all of the skills that they needed to survive and prosper, provided that the Sinn Achai agreed to live on the surface of Jacurutu during their training.
All of the Sinn Achai Eldar trained with the Exodites in turns, spending years on the surface until they were as accomplished survivalists as their hosts. The soft and decadent Sinn Achai were transformed beyond recognition, and they returned to the stars. Throughout their travels, they maintained their strong connection to the Jacurutu.