The Infernal Lexicon

This book contains a primer in the Infernal language. Anyone who spends a long rest reading it may choose to lose one existing known language proficiency and replace it with proficiency in Infernal instead, but doing so "shall be recorded among the archdukes of Hell for accounting during the last days."

The book also details forms of etiquette to be used with specific greater devils and archdevils in order to ingratiate oneself with such beings.

The book also includes a Scroll of Protection Vs. Fiends.

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SCROLLS OF PROTECTION

Usable by any class, not just a spellcaster. Each scroll of protection works against a specific type of creature as listed.

Using an action to read the scroll encloses you in a invisible barrier that extends from you to form a 5-footradius, 10-foot-high cylinder. For 5 minutes, this barrier prevents creatures of the specified type from entering or affecting anything within the cylinder.

The cylinder moves with you and remains centered on you. However, if you move in such a way that a creature of the specified type would be inside the cylinder, the effect ends.

A creature can attempt to overcome the barrier by using an action to make a DC 15 Charisma check. On a success, the creature ceases to be affected by the barrier.