“Regalia” comes directly from the same medieval Latin word that means “royal privileges.” The earliest English use of the word applied to actual royalty, but the usage broadened to include any ornamentation worn to indicate status. For example, academic regalia includes the gowns, caps, hoods, and medals worn during graduation ceremonies. Technically, “regalia” is a plural noun (as “regalis” is the singular in Latin), but there is no singular form in English. You can use it with either a plural or a singular verb because it works like a collective noun, such as “government” or “staff” (but using the plural form is mostly in British English, and Americans tend to use the singular in all situations).