From Lexicanum

The Black Library of Chaos, also known as just Black Library, is the Eldar's repository for forbidden lore. Much of the Eldar's ancient knowledge was lost during the Fall; the Library is a surviving source of Eldar knowledge concerning Chaos.

The Black Library is spoken of by Eldar as a dark craftworld existing within the Webway, far beyond the reach of the Imperium. Within are collected tomes and writings describing the Eldar studies of the warp. These writings 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 Guardian-scribes, who collate and transcribe the knowledge of the Library, a task they have carried out since the Fall. They maintain a vigil over the Library. It contains not just Eldar knowledge of Chaos; any knowledge of Chaos is taken and kept within the Library, such as the copy of the Book of Magnus. It 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 Library is known to few, and even fewer are allowed access to it. The Library's mind defends itself, barring the weak and corruptible. Only those who have mastered the Chaos within themselves are able to enter. Only two groups come and go at will: the Human Illuminati and the Solitaires of the Eldar Harlequins. The Inquisitor Czevak gained entry once, but had to prove himself worthy. Ahriman of the Thousand Sons seeks entrance to the library in his vain attempt to understand the ever changing Tzeentch, Lord of Change. Inquisitor Jaq Draco also managed to find a way into the Black Library and steal the Book of the Rhana Dandra, aka the Book of Fate.

Sources

• Warhammer 40,000: Compendium
• Realm of Chaos: Slaves to Darkness
• Realm of Chaos: The Lost and the Damned
• Harlequin (Novel), Ian Watson, Black Library Publishing

See also

• Black Library Publishing