Where does it say that? Eldar codex says:
Nothing about Slaanesh being involved at all. For Slaanesh to claim his soul , every single one of his spirit stones would have to be destroyed. As it stands, a handful of them still function.
I was reading Lexicanum, the sources listed were
* Fulgrim by Graham McNeill, Black Library, 2007.
* Heroes and Villains of the 41st Millennium: Eldrad Ulthran, Farseer of Ulthwé by Phil Kelly, White Dwarf 286.
* Codex: Eldar (4th Edition), Codex: Eldar (3rd Edition)
During the Thirteenth Black Crusade, the Eldar of Ulthwé fought against the forces of Chaos, and Eldrad was at the forefront, knowing he would not live to see the end of the conflict. Eldrad attempted to retake a Blackstone Fortress, but the fortress was already occupied by Slaanesh, the Great Enemy. As Eldrad realised his folly, his soul was devoured, and the greatest Farseer of the Eldar was lost, however a handfull of waystones he had created still remained active causing some Eldar to believe that he's still alive but trapped in the warp.
I myself prefer what you cited