Complex sentence: Amidst growing opposition, Elagabalus, just 18 years old, was assassinated and replaced by his cousin Severus Alexander on 11 March 222, in a plot formulated by his grandmother, Julia Maesa, and carried out by disaffected members of the Praetorian Guard.
Split into simpler sentences: Amidst growing opposition, Elagabalus, just 18 years old, was assassinated and replaced by his cousin Severus Alexander on 11 March 222. The plot was devised by his grandmother, Julia Maesa, and carried out by disaffected members of the Praetorian Guard.