The Roman d'Alexandre en prose (Prose Alexander-Romance) is one of many medieval "Alexander romances" relating the adventures of Alexander the Great, which were by then greatly elaborated with fantastical additions to the historical accounts. Alexander was one of the medieval "Nine Worthies," and his journeys eastward—and most especially the strange and exotic people and animals he encountered there—were treated in a number of different texts in a variety of genres. Based on the relatively large number of surviving manuscripts, as well as the deluxe quality of many of these productions, the Old French Roman d'Alexandre en prose can be considered the most popular and successful vernacular prose treatment of the legend.
In the mid-tenth century, Archpriest Leo of Naples translated into Latin a second-century Greek Alexander romance falsely attributed to Callisthenes. This new translation was later supplemented by other material (from sources including Orosius's Historia adversus paganos) and, in its expanded form, came to be known as the Historia de preliis. It is this Latin version that was, in its turn, freely translated into the Old French text known as the Roman d'Alexandre en prose (Alexander Romance in Prose), borrowing at times from other sources, including the verse Roman d'Alexandre. The prose romance dates to the thirteenth century. There are three major recensions of the text, where subsequent editors either added supplements or made excisions.
Nectanebus the magician and astrologer is the king of Egypt, but the country is attacked by the Persians and Nectanebus is sent into exile. He finds a new home at the court of Philip, the king of Macedon. The king is away, leaving his wife Olympias behind. Nectanebus prophesies to Olympias that the god Amon will visit her in a dream, and conceive a son. Nectanebus himself then proceeds to make the prediction true by coming to the queen at night disguised as a dragon. The queen becomes pregnant, and is initially concerned about the anger of Philip when he returns. But Philip has himself had a prophetic dream, foretelling that his wife will give birth to a boy, conceived by a god, who will go on to be a great conqueror. He therefore accepts the illegitimate child as his own.