Codex: Chaos Space Marines (2002), p6 — The Eye of Terror

At the heart of the Eye of Terror lie the crone worlds: the original homelands of the Eldar people. These worlds are the domain of daemons and lunatics but are also the only source of the material used by the Eldar to create their spirit stones: repositories for the Eldar soul to shield it from the eternal thirst of Slaanesh. Some Eldar still travel to the crone worlds in search of this material, which is said to have been formed of the raw stuff of chaos at the moment of the Fall. Few of these individuals ever return.