A Forbidden Tome represents a floor of a Sanctum, when a floor is complete the next floor is generated as a tradable item that keeps all your progress, Resolve, Boons & Afflictions. You can choose to play your Sanctum or Trade it away depending on the rewards or dangers ahead.
Take this Forbidden Tome to Divinia.
Forbidden Tome is a misc map item. It can be given to Divinia to allow entry into the Forbidden Sanctum.
Forbidden Tome can drop anywhere.
After meeting Divinia in Act Ten, you’ll find Sanctum items while playing maps in the endgame. These tradeable items each represent a whole floor of the Sanctum. Upon successfully completing that Sanctum floor, the next floor is generated as a tradeable item, with all your Boons, Afflictions and Resolve states built-in.
GGG has rebalanced The Forbidden Sanctum to provide more variety in the mechanics and monsters you encounter. Most notably, your primary character defences are now able to protect your resolve. Relics have also been rebalanced and are now tradeable.