Alexander Severus was adopted as son and caesar by his slightly older and very unpopular cousin, the emperor Elagabalus at the urging of the influential and powerful Julia Maesa — who was the grandmother of both cousins and who had arranged for the emperor's acclamation by the Third Legion. On March 6, 222, when Alexander was just fourteen, a rumor went around the city troops that Alexander had been killed, triggering a revolt of the guards that had sworn his safety from Elegabalus and his accession as emperor.
The running of the Empire during this time was mainly left to his grandmother and mother (Julia Soaemias). Seeing that her grandson's outrageous behavior could mean the loss of power, Julia Maesa persuaded Elagabalus to accept his cousin Alexander Severus as caesar (and thus the nominal emperor-to-be). However, Alexander was popular with the troops, who viewed their new emperor with dislike: when Elagabalus, jealous of this popularity, removed the title of caesar from his nephew, the enraged Praetorian Guard swore to protect him. Elagabalus and his mother were murdered in a Praetorian Guard camp mutiny.