The family of his mother, Julia Soaemias, were hereditary high priests of the god Baal at Emesa (in ancient Syria), worshiped in that locality under the name Elah-Gabal (thus Elagabalus). The emperor Caracalla (211–217), Bassianus's cousin, was murdered in 217 and replaced by the praetorian prefect Macrinus. Bassianus's mother and his grandmother, Julia Maesa, won the support of the nearby troops by passing him off as an illegitimate son of Caracalla, whose name he assumed for official purposes. Soon thereafter (218) the remainder of the Eastern armies deserted Macrinus.

Acknowledged as emperor by the Senate, Bassianus, by virtue of his priestly function, became generally known as Elagabalus. He tried to impose the worship of Baal upon the Roman world, executed a number of dissident generals, and pushed into high places many favourites distinguished by personal beauty and humble and alien origins. The first crisis of his regime occurred when he divorced his wife to marry the Vestal Virgin Aquilia Severa and declared this union to be a “sacred marriage,” like Baal's mating with Juno Caelestis. He was persuaded by Julia Maesa, the real power in the government, to adopt his docile cousin Alexander (Severus Alexander) as his son and heir (221) and to divorce Aquilia in favour of a match with a descendant of Marcus Aurelius, Annia Faustina. When Elagabalus changed his mind and sought to depose Alexander and resume his relationship with Aquilia, the Praetorian Guards mutinied, killed Elagabalus and his mother, and made Alexander emperor.