Daemonomicon (Patreon)

Wondrous item, very rare

This leather tome is filled with demonic lore and occult secrets, which are written in Abyssal. You can spend 10 minutes reading through its pages to learn information about an aberration, demon, or cult. At the end of that time, make an Intelligence saving throw. The DC is equal to 10 + half the aberration's or demon's challenge rating (rounded up), or 16 if you used it to learn information about a cult. On a failed save, you take 4d6 psychic damage, which can't be reduced or prevented in any way, and learn nothing from the book. You have advantage on the saving throw if you're a warlock with an eldritch or fiendish patron. On a successful save, you learn up to three of the following pieces of information of your choice about an aberration or demon:

• Up to three of its ability scores of your choice
• Any damage resistances, immunities, or vulnerabilities it has
• Any condition immunities it has
• Its armor class
• Its hit point maximum
• Its challenge rating

Alternatively, if you read the book to learn about a cult and succeed on the saving throw, you have a +10 bonus to the next ability check you make to uncover such information related to the cult.

Curse. This book is cursed, and failing the Intelligence saving throw extends the curse to you. While cursed, you are unwilling to part with the book, keeping it within reach at all times, and you have disadvantage on attack rolls against aberrations and demons and on saving throws against their spells and special abilities. If you're a warlock, you also learn the following spells, which don't count against the number of spells you know: banishment, bestow curse, hellish rebuke, and scorching ray. Charisma is your spellcasting ability for these spells, which you know for as long as you remain cursed. If you have a Book of Shadows, this book replaces it, and whenever you have to make an Intelligence saving throw when reading the book, you can make a Charisma saving throw instead.

"I am obsessed with this book—I just can't put it down. It's like a little pet project to learn everything I can about these creatures. Some are quite cute, you know. I mean, look at the pictures!"