Serapis was born from political necessity. When Ptolemy I Soter, one of Alexander the Great's generals, seized Egypt in 305 BCE, he faced a kingdom of Egyptians who worshipped Osiris and a new population of Greek settlers who worshipped Zeus, Hades, and Dionysus. Alexander himself had tried to use Amun, the ram-headed god of Thebes, as a unifying deity, but Amun was too deeply Egyptian to appeal to the Greeks of the Delta. Ptolemy needed a god who could be both Egyptian and Greek at the same time, a deity whose parentage was foreign enough to be acceptable to the conquerors and ancient enough to command the reverence of the conquered. He found his answer in Memphis, where the cult of Osiris-Apis, the dead Osiris fused with the sacred Apis bull, had existed for centuries.

The defining myth of Serapis's creation was recorded by Plutarch, the Greek biographer and philosopher, in his treatise On Isis and Osiris. According to Plutarch, Ptolemy had a dream in which he saw a colossal statue of an unknown god that commanded him to bring it to Alexandria. The king had never seen this statue and did not know where it was, but when he told his friends of the vision, a well-travelled man named Sosibius said he had seen just such a statue at Sinope, a Greek city on the southern shore of the Black Sea. Ptolemy sent two agents, Soteles and Dionysius, who after great difficulty and with what Plutarch calls the help of providence succeeded in stealing the statue and bringing it to Alexandria. When the statue arrived, two religious experts examined it. One was Timotheus, a member of the Eumolpidae, the ancient Athenian family from whose members the hierophant of the Eleusinian Mysteries - the most secret and sacred rites of Greece - had been chosen since before recorded history. The other was Manetho, the Egyptian priest and scholar who had already written the history of the Egyptian kings in Greek. Together they pronounced the statue to be Serapis, identifying it with the Egyptian Osiris-Apis and the Greek Pluto, the king of the underworld. The statue certainly did not bear the name Serapis when it came from Sinope, Plutarch notes, but after it was conveyed to Alexandria it took the name which Pluto bears among the Egyptians.

Whether the Sinope story is literally true or whether the statue came from a place called Sinopeion near the existing Serapeum at Memphis, the result was the same: Ptolemy built the greatest temple in the Greek world, the Serapeum of Alexandria, and placed Serapis at its centre. The temple was constructed on the hill of Rhakotis, the oldest part of Alexandria, and rose in grandeur through the reigns of Ptolemy II and Ptolemy III until it became not merely a sanctuary but a symbol of the city's intellectual and cultural supremacy. The Serapeum housed a portion of the Library of Alexandria, the greatest collection of books in the ancient world, and its courts were filled with philosophers, astronomers, and pilgrims from every corner of the Mediterranean.

Serapis was depicted as a powerful, bearded man enthroned like a Greek Zeus or a Roman Jupiter, but his attributes were a mixture of Egyptian and Greek symbolism. On his head he wore the modius, a basket-shaped grain measure that signified his role as a god of fertility and the harvest, and that also connected him to the underworld because grain must be buried in the earth to grow. In his hand he held a sceptre of rulership. At his feet lay Cerberus, the three-headed hound who guarded the gates of Hades in Greek myth, a creature so terrible that even the gods feared him. At the base of the statue coiled a serpent, the Egyptian uraeus, the rearing cobra that spat fire at the enemies of the sun god Re. The combination was deliberate: Serapis was a god who ruled the dead like Osiris, the sky like Zeus, and the fertile earth like Dionysus. He was accompanied in his cult by Isis, his consort, and by Harpocrates, the Greek form of the Egyptian Horus the Child, depicted as a small boy with his finger to his lips in the gesture of silence.

The cult of Serapis spread with the speed of empire. Under the Ptolemies it was disseminated as deliberate policy, and under the Roman Empire it became one of the most popular religions in the Mediterranean. At Rome, Serapis was worshipped in the Iseum Campense, the sanctuary of Isis built during the Second Triumvirate in the Campus Martius. The Roman cults of Isis and Serapis gained enormous popularity in the first century CE, particularly after the emperor Vespasian experienced what he attributed to their miraculous agency while he was in Alexandria in 70 CE. From the Flavian dynasty onward, Serapis appeared on imperial coinage alongside the reigning emperor, a visual statement that the god and the state were partners in power. Pausanias, the Greek traveller who wrote a guide to the monuments of Greece in the second century CE, noted Serapeia - temples of Serapis - on the slopes of Acrocorinth above Corinth and at Copae in Boeotia. The god had become universal.

But universal gods make universal enemies. By the fourth century CE, Christianity had become the dominant religion of the Roman Empire, and the old pagan cults were targeted for destruction. In 391 CE, the emperor Theodosius I issued a decree outlawing pagan worship and closing the temples. In Alexandria, the Christian patriarch Theophilus obtained legal authority over a temple of Dionysus and converted it into a church, then paraded the pagan cult objects - including the phalli of Dionysus - through the streets in mockery. The pagans rioted, killed Christians, and barricaded themselves inside the Serapeum, the last great stronghold of paganism in the city. Theodosius offered the pagans pardon but ordered the destruction of the temple and its images. According to the church historian Rufinus, a Christian soldier seized a double-edged axe and struck the jaw of the old wooden statue of Serapis, hacking at the worm-eaten wood blackened by centuries of sacrificial smoke. The head fell, then the feet, then the limbs were ripped from the torso with ropes and burned in a fire before the eyes of the Alexandrians. The torso itself was dragged to the amphitheatre and burned there as a final act of contempt. A brick at a time, the temple was dismantled, its gold and precious marbles stripped away, and crosses were erected on the ruins. The ancient demon Serapis, as Rufinus called him, was finally destroyed.