Diadumenian was born on 14 September 208, named Marcus Opellius Diadumenianus, to Macrinus, the Praetorian Prefect and future emperor of Berber origin,[2] and his possibly fictitious wife Nonia Celsa.[3] Macrinus declared himself emperor on 11 April 217, three days after Emperor Caracalla was assassinated. Shortly after, Diadumenian was elevated to caesar at Zeugma, while his guard was escorting him from Antioch to Mesopotamia, to join his father.