The Black Library of Chaos (also known simply as the Black Library) is the secret EldarCraftworld that serves as the Eldar's repository of forbidden lore concerning the Ruinous Powers that exists somewhere within the labyrinthine passages of the Webway. The Eldar Craftworlds became the only surviving sources of their ancient knowledge of sorcery and the Ruinous Powers of Chaos after the Fall of that race to the birth of Slaanesh. As the Craftwords have drifted apart, this knowledge has consequently become fragmented, and as some Craftworlds have become lost over the millennia, more precious Eldar knowledge has been lost with them. The Black Library is governed by a body of the wisest Eldar Farseers drawn from all of the Craftworlds known as the Black Council. The Black Council is the closest the fractious Eldar race has come to an advisory body that looks after the interests of the entire race.
The Eldar know more of Chaos than mankind ever will, and still more was lost during the Fall. A single source of Eldar knowledge has remained untouched and inviolate since the Fall. In a hidden location, some say at the very centre of the Webway -- the Eldar’s network of stable warp tunnels that crosses the galaxy -- rests the tomes, books, scrolls, and codices describing the Eldar’s complete and extensive knowledge of the Warp. The forbidden lore of the Black Library describes the blandishments, influences, forms, creatures, perils, promises, and horrors of Chaos.
The existence of the Black Library is known to only a few and entry is allowed to even fewer individuals. The library defends itself against the weak and those who would misuse its knowledge by refusing entry to all except those who have acknowledged and tempered the Chaos within themselves. The immature, who are still vulnerable to the promises and seductions of Chaos, find that they are unable to pass through its gateway. As a result, only the Solitaires of the Eldar Harlequin troupes are allowed to come and go freely, although some say that a human, perhaps an Inquisitor, has also been allowed to do so.