The Malefica Occultum

Known colloquially and charmingly as the Book of Vile Darkness, this black book is a compilation of the writings and musings of some of the most dark and malignant minds the world has to offer.
Originally, it was the grimoire of the archmage of legend, Vecna. In it, he inscribed the details of rituals and incantations he gleamed from his studies of the outer realms. Within it, he recorded the means by which to achieve life unending through black magic, how to capture and coerce even the most powerful beings into servitude, spells that could twist the minds of entire armies, and much, much more. As Vecna's power grew, so to did his connection with his book, effectively tethering part of his very soul between its pages. And when he passed, the book was passed on to his most devoted disciples, who poured over their old master's texts with avarice and gluttony. They added their own writings and discoveries to the book while all the while becoming more twisted and reviled as the book fed on their knowledge.
Over the centuries, the book has persisted; often being coveted by those who sought forbidden and esoteric knowledge. And those that found it would add new knowledge to the book, continuously expanding its reserves of magic and lore. Over time, it became so saturated with evil that those unprepared to read it (or worse, who didn't even know what it was) found their minds quickly driven to utter insanity. 
And yet, despite the negative energy that inundated its every page, there was pragmatic use for the book. Throughout history, there have been many a time where an otherwise good natured scholar or mage would brave the madness-inducing secrets of the Malefica Occultum in order to learn a hidden weakness or the true name of an otherwise unbeatable fiend. Most recently, in 1218 EP, the lich Shepherd claimed the book in order to find a means of capturing and binding Cataclysm before he wrought untold destruction onto the world.

Mechanics & Inner Workings

Ultimately, the book exists as a means of chronicling and sharing knowledge, but at its very core, the Malefica Occultum was once akin to a wizard's spellbook, and thus can be used to channel magic and cast spells. It also imbues those who read it with an enhanced capacity for knowledge and insight; in effect aiding its reader understand it. 
Whether it was originally crafted this way by Vecna or as merely a byproduct of the contents infused in it, the Malefica Occultum can be outright deadly to read for those not of the correct disposition. Those who are unprepared to act on the writings within may find themselves destroyed by the evil, negative energy the book radiates. Even those that can withstand this effect find themselves physically aged just by attuning with the book, and that is only the first of the physical deformities reading the book inflicts. Those that read it fully often develop glossy black eyes and became physically ill and diminished. Mentally, very few readers walk away from the book unchanged, with many developing some form of long-term madness. 
Finally, the aura of the book allows its reader to exert influence over other creatures of the same nature. Those that have absorbed the books contents can command undead and fiendish creatures like a general marshalling troops to their cause.

Significance

This book falling into the "wrong" hands has been the cause of numerous wars and blights upon humanity. Its appearance in the canon of history is always considered a black mark, whether it directly caused the woes that haunted the land or because it was needed to stop something worse.

Item type

Book / Document

Creation Date

Circa 2500 AC

Rarity

The Malefica Occultum is the one and only copy of Vecna's writing (and those that added to it). Any attempts to copy the words within have failed and usually resulted in the death of the author attempting it.