Mordenkainen's Lost Notebook

Fool’s Speech

3-level Transmutation

You and up to four creatures who are touching each other when the spell is cast can speak and understand a secret language that is unintelligible to others. A comprehend languages or tongues spell will not decipher the language.
At higher levels: At higher levels: When you cast this spell using a spell slot of 4th level or higher, the duration increases by 1 hour for every slot level above 3rd.