The Black Library of Chaos (also known simply as the Black Library) is the secret Eldar Craftworld 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 closet 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.
The Black Library is the only source of the Eldar's knowledge concerning Chaos that has remained untouched. It was created by the Eldar deity Cegorach, better known as the Laughing God, though for what purposes the God created this repository can only be guessed. The Black Library is spoken of by the Eldar as a dark Craftworld existing within the Webway, far beyond the reach of the Imperium of Man. Within are the collected tomes and writings describing the Eldar's studies of the Immaterium and all the entities that live within it. These writings also describe all aspects of Chaos: its perils, promises and horrors. The Library is enclosed within a nearly impenetrable psychic barrier, and is watched and maintained by its Eldar Guardian-Scribes, who collate and transcribe the knowledge of the Black Library, a task they have carried out since the Fall. The Black Library contains not just the Eldar's knowledge of Chaos; any new knowledge of Chaos gained by any intelligent race of the galaxy is taken and kept within the Library, such as a copy of the Book of Magnus, the potent tome of sorcery created by the Thousand Sons Traitor Legion's Primarch or the Book of Lorgar written by the Primarch of the Word Bearers Traitor Legion that sought to spread the teachings of the Ruinous Powers, both before the Horus Heresy.
The Black Library is as much a source of knowledge for those who fight Chaos as it is a vault to keep dangerous knowledge hidden and protected from those who would misuse it. The existence of the Black Library is known to few, and even fewer are allowed access to it. The Library's collective psychic mind defends itself, barring the weak and corruptible. Only those who have mastered the temptations of Chaos within themselves are able to enter. Only two groups come and go at will: the human Illuminati and the Eldar Solitaires, the most dangerous of the Harlequins. The Imperial Inquisitor Bronislaw Czevak was granted entry by the Black Council once to study the accumulated lore concerning Mankind's interaction with Chaos, but had to prove himself worthy of the honour. The Chaos Sorcerer Ahriman of the Thousand Sons Traitor Legion sought entrance to the Library in his vain attempt to better understand his master, the mercurial Tzeentch, the Changer of the Ways and the Chaos God of Sorcery. So far, the Black Library and its guardians have managed to repel this particularly troublesome would-be invader. Inquisitor Jaq Draco also managed to find a way into the Black Library and steal the Book of the Rhana Dandra, known alternatively among Imperial scholars as the Eldar Book of Fate.
The Black Library is a "dark" Craftworld eternally travelling through the Webway. Its location and route are secret, hidden from all but the Harlequins and the members of the Black Council. Like all Craftworlds, the vessel is conscious and psychoactive, hiding itself from all psychic probes, while simultaneously repelling Warp entities and intrusions. It is an enormous construct with a scorched, crystalline appearance, covered in vanes and flourishes but with the typical flowing grace of Eldar architecture. The interior is an intentional labyrinth with all manner of convoluted passages, corridors, and chambers. Bubble vaults full of stasis caskets containing forbidden lore and dangerous artefacts hang from the superstructure, whilst inside and above are myriad citadels, obelisks, monoliths, and spires where the rest of the Craftworld's arcane contents are kept.
Among the varied structures present within the Black Library is the Wraith Tower, the section of the Library devoted to humanity's interaction with Chaos, the accumulated lore of the Imperial Traitors. The psychic emanations of the Webway are known to have a regenerative effect upon human tissue, which greatly benefited the human Inquisitor Czevak. Several standard years after he was granted permission to study at the Black Library, a much younger-looking Czevak returned to realspace and the Imperium, no longer needing the assistance of his cryogenic suspension suit to easily move his once-aged body around.
- Warhammer 40,000: Compendium (2nd Edition)
- Realm of Chaos: Slaves to Darkness, p. 215
- Realm of Chaos: The Lost and the Damned
- Harlequin (Novel) by Ian Watson
- Xenology (Background Book)
- Atlas Infernal (Novel) by Rob Sanders