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Diadumenian (Latin: Marcus Opellius Antoninus Diadumenianus Augustus) (September 14/19, 208 – 218), was the son of the Roman Emperor Macrinus, and served his father briefly as Caesar (May 217–218) and as Augustus (in 218). He was so named due to being born with a caul that formed a 'diadem'.
Diadumenian was born on 14 September 208 or, according to Historia Augusta, on September 19 because he shared the same birthday with the Emperor Antoninus Pius. His mother was Empress Nonia Celsa, although little is known of her: she is only mentioned by the Historia Augusta. He was born Marcus Opellius Diadumenianus, but his name was changed and added Antoninus to solidify connection to the family of Marcus Aurelius as done by Caracalla.
Diadumenian had little time to enjoy his position or to learn anything from its opportunities because the legions of Syria revolted and declared Elagabalus ruler of the Roman Empire. When Macrinus was defeated on 8 June 218, at Antioch, Diadumenian's death followed his father's.