Answer
Julia Maesa, the grandmother of Elagabalus, devised the assassination plot against him. Julia Maesa was a powerful woman in the Roman Empire, and she used her influence to get her son, Severus Alexander, appointed as emperor. She then used her influence to get Elagabalus appointed as co-emperor. However, Elagabalus was a controversial figure, and Julia Maesa eventually decided that he was a threat to her son's power.