User blog comment:NecrusIV/Requesting approval: El'Jonson Heresy/Empire of Ynnead/@comment-9465750-20120822075936/@comment-1536954-20120822095105

Quoting lexicanum:

"They believe that when the Infinity Circuits hold all the spirits of their race, all of the Craftworlds will unite into one Infinity Circuit, and the collective spirits of the Eldar will join to form a new Power in the Warp that will battle and subdue Slaanesh, so that Eldar spirits may once more be able to merge with it and form a single, balanced entity. By doing so, if such a thing is possible, they hope that this will allow the Eldar race to be reborn into a better form."

I've changed this slightly to that Ynnead has been born into a material form and has reanimated the Eldar race in "better form" (Their star-spanning empire).