Wondrous Item

Legendary Requires Attunement

Mark of Darkness. The mark of darkness grants you advantage on Charisma (Persuasion) checks made to interact with evil creatures and Charisma (Intimidation) checks made to interact with non-evil creatures.   Command Evil. While you are attuned to the book and holding it, you can use an action to cast the dominate monster spell on an evil target (save DC 18). You can’t use this property again until the next dawn.   Dark Lore. You can reference the Book of Vile Darkness whenever you make an Intelligence check to recall information about some aspect of evil, such as lore about demons. When you do so, double your proficiency bonus on that check.   Dark Speech. While you carry the Book of Vile Darkness and are attuned to it, you can use an action to recite words from its pages in a foul language known as Dark Speech. Each time you do so each non-evil creature within 15 feet of you takes 3d6 psychic damage.   Other Properties.

• Creatures with a challenge rating of 0, as well as plants that aren't creatures, drop to 0 hit points when within 10 feet of The Book of Vile Darkness.
• While you are attuned to The Book of Vile Darkness, animals within 30 feet of you are hostile toward you.
• While attuned to The Book of Vile Darkness, all holy water within 10 feet of you is destroyed.
• While attuned to The Book of Vile Darkness, nonmagical flames are extinguished within 30 feet of you.
• While attuned to The Book of Vile Darkness, your walking speed increases by 10 feet.
• While attuned to The Eye of Vecna, you gain proficiency Investigation and Religion. 
• While attuned to The Book of Vile Darkness, you have resistance force damage.

The contents of this foul manuscript of ineffable wickedness are the meat and drink of those in evil’s thrall. No mortal was meant to know the secrets it contains, knowledge so horrid that to even glimpse the scrawled pages invites madness. Most believe the lich-god Vecna authored the Book of Vile Darkness. He recorded in its pages every diseased idea, every unhinged thought, and every example of blackest magic he came across or devised. Vecna covered every vile topic he could, making the book a gruesome catalog of all mortal wrongs. Other practitioners of evil have held the book and added their own input to its catalog of vile knowledge. Their additions are clear, for the writers of later works stitched whatever they were writing into the tome or, in some cases, made notations and additions to existing text. There are places where pages are missing, torn, or covered so completely with ink, blood, and scratches that the original text can’t be divined. Nature can’t abide the book’s presence. Ordinary plants wither in its presence, animals are unwilling to approach it, and the book gradually destroys whatever it touches. Even stone cracks and turns to powder if the book rests on it long enough.You can then freely modify the book’s contents, provided that those modifications advance evil and expand the lore already contained within.