In the early 3rd century AD, the Roman Empire was teetering on the edge of its epic fall. On March 8, 217, the despised and ruthless Emperor Caracalla was assassinated as he was relieving himself against a roadside tree during a long overland ride. Apparently, the Emperor wanted some privacy and, in his moment of vulnerability, one of his guards did him in with a single thrust of a dagger. The assassin, Julius Martialis, was immediately shot with an arrow by a Scythian bodyguard, and so did not outlive the Emperor for very long.
However, Caracalla, while reviled by the general public, was very popular with the soldiers, and this was a time when the Roman army wielded a great amount of political power. Adventus, Caracalla's head of the military, declined the position of Emperor, and instead encouraged the promotion of Opilius Macrinus, a prefect of the Praetorian Guard who controlled the Empire's civil and financial affairs. Macrinus, a gifted businessman by nature who had trouble understanding the military, and a foreigner by birth who rarely left Syria, quickly fell out of favor with the Roman army. Despite Macrinus's skill in financial matters, he was reluctant to engage in battle, and the soldiers disrespected him; before long they began looking around for a worthy successor to Caracalla.
Their choice was perhaps the most enigmatic Roman Emperor in history. Varius Avitus Bassianus, the son of Caracalla's Syrian cousin, was all of fourteen years old, and was by all accounts a capricious, prancing homosexual who worshipped El-Gabal, the Syrian sun god, and so under normal circumstances would probably not be the favored son of the Roman military elite. However, Julia Maesa - Caracalla's aunt and Avitus's grandmother - renamed the boy Marcus Aurelius Antoninus, and began spreading the false news that he was Caracalla's illegitimate son. Soldiers still loyal to Caracalla - commanded by Avitus's eunuch Gallys, who had absolutely no battlefield experience - backed the young boy, and at the Battle of Antioch on June 8, 218, they defeated an army of veteran Praetorians under Macrinus's command after the "False Antoninus" charged boldly into the enemy ranks, swinging wildly and inspiring his men to follow. Macrinus and his son were both caught and immediately executed.
The reign of "Antoninus" was bizarre and unrestrained. He married five times in four years, once to a woman whose husband was one of the many people he had killed upon taking office, and in between wives he had defiled a Vestal Virgin with the intention of making godlike children. He had the palace statue of Jupiter decapitated and replaced with the head of El-Gabal, and he circumsized himself as a display of purity. He performed rites and staged parades dedicated to his alien deity in public, all of which was seen as blasphemous by the public. He had a habit of standing naked in a palace doorway, propositioning passers-by and prostituting himself to them. He cross-dressed virtually every day, and as a woman he "married" a slave named Hierocles, referring to himself as "his Queen".
None of this sat well with the army who had installed him on the throne. In March of 222, Julia Maesa, the same grandmother that had installed him, decided she would replace him with his younger cousin Severus Alexander. In response, Antoninus stripped Alexander of his titles, a riot broke out, and the Praetorian Guard called for Antoninus. Suddenly, the Emperor realized his time was up, and he - still small of frame and only 18 years old - climbed inside a chest with the hopes of escaping. He was, however, found, and the Guards cut off his and his mother's heads, and threw their bodies in the Tiber River.
After his death, Marcus Aurelius Antoninus became known either as Elagabulus - the Latinized version of his Syrian deity's name - or Tiberinus, in mocking memory of the river into which he was thrown. He was so hated that he was issued a damnatio memoriae, the results of which were that his name and likeness were completely erased from all records, accounts, and statues, in an attempt to eliminate entirely from history.
Links and Sources:
Roman History, Volume IX, by Cassius Dio, available here.
The Historia Augusta, published in the Loeb Classical Library, 1924, available here.
The Decline and Fall of the Roman Empire, by Edward W. Gibbon, available here.
Source pages for photos of busts of Caracalla, Macrinus, and Elagabalus.