Marcus Opellius Antoninus Diadumenianus was the son of the Roman emperor Macrinus and served as his co-ruler for a brief time, from mid May to 8 June 218. Macrinus was praetorian prefect for Caracalla but arranged Caracalla's assassination and seized power on 11 April 217. Shortly after, the eight or nine-year-old Diadumenian was elevated to caesar while his guard was escorting him from Antioch to Mesopotamia to join his father. In May 218, after Macrinus concluded an unfavorable truce with the Persians, the Syrian legions, inspired by Caracalla's aunt, Julia Maesa, revolted in support of her son Elagabalus. Diadumenian was then made co-emperor. Macrinus was defeated in the Battle of Antioch on 8 June 218. Diadumenian was sent to the court of Artabanus IV of Parthia to ensure his safety; however, he was captured and executed along the way. Macrinus was also captured and executed. Their heads were sent to Elagabalus, who kept them as trophies. The Senate declared father and son enemies of Rome and had their names struck from records and their images destroyed — damnatio memoriae.