Actions that breach a society's sense of morality may cause outrage or amusement; actions that undermine a society's sense of mythology may cause it to doubt its own validity. When the Emperor Caligula fell in love with his horse Incitatus, it was seen as an eccentric joke; distasteful, but nothing to get worried about. But when in CE220, the Emperor born Varius Avitus Bassianus married the Vestal Virgin Julia Aquilia Severa, the transgression convulsed Rome with shockwaves of shame and alarm. For the virginity of the Vestals was crucial to Rome's sense of its own survival.

Bassianus was the hereditary high priest of the god Elagabal (god of the mountain), whose cult centre was at Emesa (modern-day Hims) in the Syrian desert. As Emperor of Rome, he was crowned Emperor Marcus Aurelius Antoninus, though he is more commonly known as Elagabalus  or Heliogabalus (a scornful nickname meaning Scoundrel of the Sun); for simplicity's sake, I shall continue to call him Bassianus. He became Emperor by means of old-fashioned behind-the-scenes politicking, following the assassination of Caracalla, his first cousin once-removed, in CE217.

Caracalla was killed while on his way to the city of Harran in northern Syria, to make a sacrifice to the moon god Sîn; although Sîn's main temple was at Ur, Harran had become an important cult centre of this ancient Mesopotamian god in the Neo-Babylonian Period. That a Roman Emperor's worship should be focussed on a god dating back to Ancient Sumer is not unusual; this was a period at which the religions of Egypt, Syria, and Anatolia were profoundly influential through the Roman Empire, and even at Rome itself. And most of these religions had roots in Mesopotamian myth. The cult of Elagabal, for instance, was influenced by that of the Babylonian sun god Shamash, while his  main festival, celebrated in mid-summer, showed great similarities with the Babylonian New Year Festival. 

Caracalla's father, Septimius Severus, had commanded a legion in Syria; his mother, Julia Domna, was the daughter of Elagabal's high priest. As Juvenal had written a century before, "The Syrian Orontes has been disgorging into the Tiber for a good while now."

It was Bassianus's grandmother, Julia Maesa, who manoeuvred him into power in CE218, ousting Caracalla's murderer and successor, Macrinus. The teenage Bassianus, however, was truly devoted to the sun god Elagabal, and refused to leave behind the conical black meteorite (baetyl) which was the sacred embodiment of the god. It was worshipped, according to Herodian "as though it were sent from heaven; on it are some small projecting pieces and markings that .... people would like to believe are a rough picture of the sun". A number of such baetyls were the objects of cults in the Levant (for instance that of the god Dusares in Arabia); an important one that survives to this day-co-opted into Islam-is the Black Stone in the Kaaba in Mecca.

Bassianus took the baetyl of Elagabal to Rome, in a flamboyant procession across Asia Minor that took over a year to complete, as the teenage Emperor insisted on sacrificing to the god at every opportunity. When at last he arrived in Rome, he immediately established a sanctuary for the baetyl, the Elagabalium, beside the imperial palace. From now on, Elagabal Sol Invictus (Elagabal the Unconquered Sun) was to be the chief god of Rome, superior even to Jupiter Capitolinus; all the other gods were mere servants of Elagabal. Each day the perfumed emperor, caked in make-up, dressed in silk robes and baggy Persian trousers, sacrificed bulls, sheep, and wine to his god, dancing and singing in the language of the Bedouins to the exotic rhythms of cymbals and timbrels.

In the way of Syrian, and indeed Egyptian, gods, Elagabal was worshipped at Emesa as the head of a triad of deities, alongside the warrior goddess Allat and the moon goddess Astarte (also called Caelestis, and identified with Venus). Some sources say that the triad was Elagabal, Astarte, and the great mother Atargatis (Dea Syria), but it seems more likely that aspects of Atargatis were simply fused with Astarte. To recreate this symmetry, Bassianus moved the Palladium-the idol of Pallas Athena supposedly brought from Troy by Aeneas-from the temple of the Vestals to the Elagabalium, to wed her to his god. And then from Carthage he imported the cult statue of Tanit-Caelestis, a form of Astarte already identified with Roman Juno, to be Elagabal's second wife.

This profound rearrangement of the divine order-replacing the Capitoline triad of Jupiter, Juno, and Minerva with Elagabal, Tanit, and Athena-was reflected in the scandalous behaviour of the new emperor, who is recorded in the uncomprehending biographies of writer such as Cassius Dio as perhaps the most perverted and cruel of all the Roman emperors. Bassianus is said, for instance, to have installed a brothel in the imperial palace, in which he prostituted himself, and also to have planned to castrate himself and pleaded with his physicians to create a vagina for him. 

However much truth there is in the lurid tales of Bassianus's behaviour (it is possible, for instance that stories of his intended or actual castration are simply mistaken interpretations of his circumcision, demanded by his god), they point to various practices that are well-attested in Eastern religions of the time. The priests of Cybele, and those of her Syrian equivalent Atargatis, castrated themselves in her honour, while temple prostitution was a feature of many oriental religions, including the worship of Isis and Astarte.

Bassianus's desecration of the temple of Vesta, by removing the Palladium, was completed by his marriage to one of Vesta's sacred virgins. The outcry was so great that he put her aside, only to re-marry the following year. Such determined flouting of Rome's most sacred traditions can only be explained by Bassianus's urgent need to shore up the worship of his own god by allying himself with the most powerful Roman divinities, and co-opting them to the worship of Elagabal. 
To the Romans, this was a vile sacrilege. To Bassianus, it was a sacred marriage. If he could not persuade his surgeons to perform a sex change he could, by effectively turning Julia Aquilia Severa from a virgin dedicated to Vesta into a sacred prostitute in the service of the Emesan triad, divert the most sacred streams of Roman worship into Syrian channels. Cassius Dio quotes him as claiming that "I did it in order that godlike children might spring from me, the high priest, and from her, the high-priestess."

His grandmother Julia Maesa, herself a thoroughly Romanized woman with little time for Syrian religion, eventually despaired of Bassianus's religious and personal excesses, and had him murdered in CE22. He was replaced by his moderate cousin Severus Alexander, whose first act as emperor was to send the baetyl of Elagabal back to Emesa.