Tome of Karebennian
This legendary guidebook is said to contain a description of how individuals can take a route through the Webway in order to reach the fabled Black Library itself. A hand written copy of the tome had been known to circulate in the galaxy and had been used by a Chaos Sorcerer of the Thousand Sons, who used it to locate a Webway portal on the planet Etiamnun III. In his haste to get away from the Imperial world, the copy of the text was left behind and recovered by the Secret Order of Psykana of the Blood Ravens Chapter who ultimately destroyed it. The true Tome of Karebennian was in the possession of the Sorceror Ahriman who used it to locate the Aracadium Librarium as well as a means of finding the Black Library.