The Black Library is supposedly the Eldar's repository for knowledge on the forces of Chaos. Much of the knowledge they had learnt during their studies of the forces of Chaos was lost during the Fall of the Eldar. The remaining knowledge of the Eldar was placed within the craftworlds and scattered throughout the galaxy. Over time, this knowledge was lost as craftworlds were lost, destroyed or taken over by enemy forces.
One, supposed, craftworld has survived untouched by alien influences. Deep within the Eldar webway, far beyond the reach of the Imperium, is the Black Library of Chaos. Within are collected tomes, books and codices describing the Eldar studies of the warp. These objects describe the blandishments, influences, forms, creatures, perils, promises and horrors of the warp and the realm of Chaos.
It is enclosed within a vast, nearly impenitrable psychic barrier that even the guiding light of the Astronomican would be hard pressed to break through. It is tended by the guardian-scribes, who collate and transcribe the knowledge within the library, a task which they have carried out since the fall. They also maintain a watchful eye over their charges, for the books themselves contain such evil that they must be monitered at all times.
The library is only known to a few people, and even less may access it. The library has a 'mind' so to speak, which decides whether an individual may access the library. The immature that are still vulnerable to the promises and seductions of chaos find they cannot enter the library, lest they fall down the wrong path. As a result, very few have seen the inside of the library, let alone read any of the books. Once a guardian has entered the library, they very rarely leave it until they die.
Only two groups may come and go at one time, the Human Illuminati and the Eldar Harlequins. One Inquisitor Czevach 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.