noun[treated as singular or plural]
mid 16th century (in the sense 'royal powers'): from medieval Latin, literally 'royal privileges', from Latin, neuter plural of regalis 'regal'
The word regalia comes from Latin and is, technically speaking, the plural of regalis. However, in the way the word is used in English today it behaves as a collective noun, similar to words like staff or government. This means that it can be used with either a singular or plural verb (the regalia of Russian tsardom is now displayed in the Kremlin or the regalia of Russian tsardom are now displayed in the Kremlin), but it has no other singular form.