The earliest Ambrosius is the man in Gildas. Around 540 the monk wrote De Excidio, the ruin of Britain, and named one leader who had not gone under in the Saxon storm: Ambrosius Aurelianus, a modest man, perhaps the last of the Roman nation, whose parents had worn the purple and were slain in the catastrophe. Under him, Gildas says, the broken Britons found their strength again and the war swung back and forth until the siege of Mount Badon. That is nearly all the fifth-century record gives. Bede copied it. Later ages called him the last of the Romans, and no one has worn the title better.

The boy at Dinas Emrys is not that man. The Historia Brittonum, the Welsh history of about 829, tells how Vortigern tried to raise a tower in Snowdonia and how each day’s work sank in the night. His magicians demanded the blood of a fatherless boy. Searchers found the child at Campus Elleti in Glamorgan, brought him to die, and were answered instead. The boy showed them a pool beneath the foundations and two dragons in it, one red and one white, and he read the sight: the red dragon is the Britons, the white is the Saxons, and the red shall drive the white out in the end. Asked his name, he said Ambrosius; asked his father, that he was a Roman consul. Vortigern gave him the fortress, Dinas Emrys, the Fort of Ambrosius. Geoffrey of Monmouth later split the tangle: he kept the name Merlinus Ambrosius for the boy seer, who becomes Merlin, and he gave the war-leader a separate life as King Aurelius Ambrosius. The dragons and the pool belong with Merlin. The command of Britain belongs here.

Geoffrey’s king is the second son of Constantine, carried to Brittany as a child when Vortigern destroyed his house, who returns with his brother Uther, burns the usurper in the tower at Ganarew, and takes the crown. He breaks Hengist at Maisbeli, where Eldol of Gloucester takes the Saxon alive and cuts off his head. He buries the British nobles murdered at the Treachery of the Long Knives in the monastery of Amesbury, and he sends Merlin and Uther to Ireland for the Giant’s Dance so that Stonehenge may stand as their monument. His death is the old enemy in a new coat. Sick at Winchester, he drinks from a cup prepared by Eopa, a Saxon hired by Vortigern’s son Pascentius and dressed as a physician. That night a comet climbs the sky with a dragon of fire. Merlin reads it for Uther as the sign of the crown and of the son to come. The Britons carry Aurelius to Stonehenge and lay him inside the circle he raised. Uther Pendragon reigns after him.

The Welsh kept him as Emrys Wledig, Ambrosius the Prince, an overlord in the generation before Arthur. Historians still ask the obvious question: a Roman-born commander who rallies the Britons in the decades the legend gives to Arthur’s youth may be Arthur’s uncle, his forerunner, or the root of the king himself. The later romances mention him only as the royal brother whose reign sets Merlin’s great works in motion. Mary Stewart’s Crystal Cave makes him the soldier-king who fathers Merlin on a Welsh princess; that is modern fiction, not a medieval source. What the sources do agree on is the hinge. Purple on one side of him, the Pendragon on the other, and Rome passing into Camelot through a single name.