Aristeas was born on the island of Proconnesus, a Greek settlement in the Propontis, the inland sea that connects the Black Sea to the Aegean. The island was famous for its white marble, but Aristeas would make it famous for something far stranger. He was a devotee of Apollo, the god of poetry, prophecy, and the sun, and he carried the god's inspiration in a form that his neighbours found terrifying. One day, while walking through the streets of Proconnesus, he entered a fuller's shop and fell down dead. The fuller, a man who cleaned woollen garments for a living, was horrified. He sent word to Aristeas' family, who came to mourn. But when they opened the door of the shop to take the body away, it had vanished. There was no sign of Aristeas anywhere on the island. His family searched in vain, and the people of Proconnesus concluded that the god had taken him.

Seven years later, Aristeas returned to Proconnesus. He walked into the agora, the public square, and told the assembled citizens that he had not been dead. His soul had left his body and travelled north, carried by Apollo beyond every land the Greeks knew. He had reached the country of the Issedones, a people who lived beyond the Scythians on the inner steppes of Central Asia. From them he learned of the peoples still further north: the one-eyed Arimaspians, the griffins that guarded gold in the mountains, and beyond them the Hyperboreans, the blessed race who lived at the back of the north wind where the sun never set in summer. He wrote all this down in a poem called the Arimaspeia, a work in three books that is now lost but whose contents were preserved in the writings of later historians.

The Arimaspeia described a chain of peoples that stretched from the familiar to the fabulous. The Issedones were the first step into the unknown, a people who honoured their dead by eating their flesh and keeping their skulls as drinking cups. Beyond them lived the Arimaspians, fierce horse-riding warriors with a single eye in the centre of their foreheads, who waged perpetual war with the griffins over gold dug from the northern mountains. The griffins themselves were not mere beasts but divine guardians, lion-bodied and eagle-headed, who nested in the Rhipaean Mountains and defended their treasure with absolute ferocity. Beyond the griffins and their gold lay the Hyperboreans, a blessed people who knew no war, no disease, and no old age, who sent sacred offerings to Apollo at Delos wrapped in sheaves of wheat, passing them from hand to hand through every nation between their land and Greece. Aristeas claimed to have seen all this not with his body's eyes but with his soul's, travelling in a state of divine possession that left his earthly form behind.

Years after his return and his second disappearance, Aristeas appeared again, this time not in his homeland but in Metapontum, a Greek city on the gulf of southern Italy. The citizens were astonished to see a man they did not recognise walking through their streets, carrying a message from Apollo. He told them that the god commanded them to build an altar in the middle of their city, and that Apollo would come to them himself if they obeyed. Then he vanished. The Metapontines, uncertain whether to believe a stranger who had appeared from nowhere, sent a delegation to Delphi, the most famous oracle of Apollo in Greece, to ask the god if the command was genuine. The Pythia, the priestess who spoke Apollo's words from a tripod over a chasm in the earth, confirmed everything. She told them that Aristeas of Proconnesus had indeed come among them, and that they should build the altar. She added a detail that made their hair stand on end: Aristeas had visited them not in the body but in the spirit, for his physical form was dead and buried in Proconnesus. When the Metapontines built the altar and made their offerings, they placed beside it a bronze statue of Aristeas holding a raven, the bird into which his soul was said to transform when it left his body. Some said they also found a bronze arrow in the temple with his name inscribed upon it, the very arrow with which Apollo had once slain the Python, the great serpent that guarded Delphi before the god claimed the sanctuary for his own.