The [[Craftworld]]s became the only surviving sources of their ancient knowledge after the [[Fall of the Eldar]]. As the Craftworlds have drifted apart, this knowledge has consequently become fragmented, and as some Craftworlds have become lost over the millennia, more knowledge has been lost with them.{{Fn|1}} The Library is the only source of the Eldar's knowledge concerning [[Chaos]] that has remained untouched. It was created by [[Cegorach]] (better known as the Laughing God), though for what purposes can only be guessed.{{Fn|2}}

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's 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 its Guardian-scribes, who collate and transcribe the knowledge of the Library, a task they have carried out since the Fall.{{Fn|1}} It contains not just Eldar knowledge of Chaos; any knowledge of Chaos is taken and kept within the Library, such as a 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.{{Cite this}}