The first 10 pages of the Demonomicon are blank. As an action while holding the book, you can target a fiend that you can see that is trapped within a magic circle. The fiend must succeed on a DC 20 Charisma saving throw with disadvantage or become trapped within one of the Demonomicon's empty blank pages, which fills with writing detailing the trapped creature's widely known name and depravities. Once used, this action can't be used again until the next dawn.
When you finish a long rest, if you and the Demonomicon are on the same plane of existence, the trapped creature of the highest challenge rating within the book can attempt to possess you. You must make a DC 20 Charisma saving throw. On a failure, you are possessed by the creature, which controls you like a puppet. The possessing creature can release you as an action, appearing in the closest unoccupied space. On a successful save, the fiend can't try to possess you again for 7 days.
When the tome is discovered, it has 1d4 fiends occupying its pages, typically an assortment of demons.