Book of Vile Darkness (DMG24)
This was the personal tome of Hieronymous, the first lich. It described his life, his ascension, the shattering of his soul, and his status as champion of Duxen. Amongst the book's powers was also the instructions for the process with which others could achieve true lichdom: to conquer the aspects of lichdom (Pestilence, Bloodshed, Desolation, and Annihilation) and take them as your own. In doing so, Hieronymous split his soul into several parts and these tenets manifested into physical forces. One could summon these either during a world-scale threat of the related tenet (Tarrasque emergence qualified for Annihilation) or complete the related ritual described in the Book of Vile Darkness (smaller scale, but required direct use of the book during it).
In addition to lichdom, the book also contained great enchantment magic, influencing the reader as it discovered more. It contributed greatly to Caen Zauber's research on a new Power Word (Control), and it successfully manipulated him into becoming more cruel and isolationist over the years he had it.
Those with the Book of Vile Darkness could use it to Plane Shift directly to the Dead Tribunal chamber of the Folding Halls, where the seats of the 4 aspects are housed. Defeated or conquered aspects would return here, to their respective seats. The book also describes the Kepenek of the Fallen: the cloak of Hieronymous granted to him by Duxen as an artifact for his champion. Ascending to true lichdom in this way would draw the attention of Duxen and would allow one the path to earn the Kepenek of the Fallen and become a champion themselves.