Codex Infernus

Wondrous item, legendary (requires attunement)

This massive, red, leather-bound book is inscribed in a language belonging to a long-extinct race of fiends predating recorded history. The interior of the book is blank and possesses infinite pages.

When a creature attunes to this book, it becomes cursed for as long as it remains attuned. While so cursed, if the creature ever visits a lower plane, the book vanishes, and the creature’s soul is consumed by fiends.

You can spend 1 continuous week of strenuous activity, during which time you do nothing else, to pore through the contents of the book, which begins revealing the details of a single fiend you have seen in person. By learning every aspect of the fiend’s existence through the book in this manner, you begin to discern its truename. This process requires you to succeed on a Wisdom (Insight) check, the DC of which is equal to 10 + the fiend’s challenge rating, learning the fiend’s truename at the end of the activity on a success. If you fail the check, you can never again attempt to learn that fiend’s truename.

Once you have learned a fiend’s truename in this manner, it can never willingly harm you and follows your every command willingly, so long as you personally addressed the fiend by its truename when you gave that command. The magic of truenames prevents you from ever forgetting the fiend’s truename, nor can the sound of the truename ever be silenced or suppressed. The knowledge to speak the truename is magical and cannot be shared with or learned by other creatures except via the methods detailed here.