A royal decree is a formal proclamation made by a monarch or other high-ranking authority figure. Synonyms for this phrase can include terms like edict, proclamation, mandate, announcement, or declaration. These terms can convey similar ideas, such as a powerful official command or order given by someone in a position of authority. Other words that could be used in place of royal decree might be royal command, imperial directive, sovereign edict, or regal proclamation. Depending on the specific context and audience, different synonyms for royal decree may be more appropriate or effective in conveying the desired message or tone.