Discover Truename

This spell is a class spell for Bards and Wizards.

Discover Truename

9th-level divination

Casting Time: 48 hours
Range: Infinite
Components: V, S, M (a secret known only to the target of the spell, and a library containing at least one hundred scholarly books on the subject of truespeech)
Duration: Instantaneous

You learn the truename of a creature you are aware of. While you know the creature’s truename you have advantage on attack rolls, saving throws, and ability checks against the creature. Legendary creatures of CR 21 or higher can attempt a Charisma saving throw, preventing you from learning their truename on a success. Once you have cast this spell, you cannot cast it again until 1 year has passed.

Ref: TPKB1 p236