Eopa was a Saxon, one of the Germanic people who had come to Britain as mercenaries and stayed as conquerors, and he served in the retinue of Pascentius, the third and last surviving son of Vortigern. Vortigern had been the king who invited the Saxons into Britain, who married Rowena the daughter of the Saxon warlord Hengist, and who had been burned alive in his tower at Ganarew by Aurelius Ambrosius and his brother Uther. Vortigern's eldest son Constans had been murdered by Pictish bodyguards, his second son Vortimer had died of wounds after driving the Saxons back to the sea, and his third son Catigern had fallen in battle. Only Pascentius remained, and he had fled to Germany to raise an army against the man who had killed his father.

Pascentius's first attempt ended in disaster. He promised vast amounts of gold and silver to the Germanic youth, built a fleet, and sailed to the north of Britain, where he began to burn and plunder. Aurelius Ambrosius, the last Roman king of Britain, the son of Constantine and the avenger of Constans, marched against him and defeated him utterly. Pascentius fled, unable to return to Germany in shame, and made for Ireland instead. There he found an ally in King Gillomanius, the Irish ruler who had no love for the Britons after Uther had stolen the Giant's Dance, the circle of enormous stones that Merlin had transported from Killaraus in Ireland to Amesbury in Britain. The two kings made a pact, fitted out a fleet, and sailed for Menevia, the city that would one day be called Swansea, intending to conquer Britain while Aurelius lay sick at Winchester.

It was then, while Pascentius and Gillomanius rejoiced at the news of the king's illness, that Eopa came to his master with a proposal. He asked what reward would be given to the man who killed Aurelius Ambrosius. Pascentius, desperate and vengeful, promised a thousand pounds of silver, his lifelong friendship, and the rank of centurion -- a commander in his army -- should he ever gain the crown. Eopa accepted. He told Pascentius that he had learned the British language, that he knew the manners and customs of the people, and that he possessed skill in physic, the art of healing. He would shave his beard and his head, don the habit of a devout and learned Christian monk, and present himself at Winchester as a physician. Once admitted to the king's presence, he would prepare a potion laced with poison, give it to Aurelius to drink, and tell him to cover himself and sleep so that the draught might work. The poison would be slow-acting, giving Eopa time to escape before the death was discovered. Pascentius swore an oath to fulfil his promise, and the bargain was struck.

Eopa arrived at Winchester laden with vessels full of medical preparations, his head tonsured, his face shaved, his body cloaked in the garb of a holy healer. The king's household received him gladly, for no one was more welcome to them at that moment than a physician who might cure their lord. Aurelius Ambrosius had been a great king, the restorer of churches, the builder of Stonehenge, the executioner of Hengist, and the burner of Vortigern, but even great kings fall sick, and even great kings grow desperate for relief. Eopa was led into the royal presence, and he promised the king that his potions would quickly restore him to full health if he would but follow the physician's instructions. Aurelius, innocent and desperate, readily agreed. Eopa prepared the mixture, secretly lacing it with poison, and handed the cup to the king. Aurelius drank it down.

The false physician then gave his final instruction. He told the king to cover himself close beneath the bedclothes and to fall asleep, so that the detestable potion might operate the more strongly. Aurelius obeyed, lying back in his sickbed and pulling the covers over himself, believing that he was on the road to recovery. The poison crept slowly through his pores and veins, a silent enemy that needed no sword and no battle cry. The king slept, and the sleep became death. Meanwhile Eopa, the accursed traitor, slipped from the chamber, then from the court, then from the city itself. By the time the servants came to wake their master, they found him dead, and when they searched for the physician who had promised to heal him, he was nowhere to be found. The man who had killed a king in his sleep had vanished into the night.

The consequences of Eopa's treachery were immediate and far-reaching. Uther Pendragon, the younger brother of Aurelius, was marching to meet Pascentius and Gillomanius when the news reached him. A great star had appeared in the heavens, a wonder of such magnitude and brightness that it trailed a ray of fire in the shape of a dragon, and Merlin the prophet had already foretold that Uther would take the name Pendragon after this sign. Uther defeated the invaders, killed Pascentius and Gillomanius, and was crowned king in his brother's place. He would go on to father Arthur, the greatest king Britain would ever know. And all of it -- the dragon comet, the crown, the sword in the stone, the Round Table -- traced back to a single Saxon who had shaved his head, mixed a poison, and told a sick man to sleep.

Eopa disappears from the chronicles after Winchester, his name never spoken again in the histories of Britain. Unlike the great villains of Arthurian romance, he has no final confrontation, no comeuppance, no scene of justice. He simply vanishes, a thousand pounds of silver heavier, a king's blood on his hands, his work done. Geoffrey of Monmouth, the twelfth-century chronicler who recorded the tale, calls him "the wicked Ambron" in one passage, a name that may be a scribal error or may hint at a lost tradition. What is certain is that Eopa achieved what no army could achieve: he killed the last Roman king of Britain without raising a weapon in open battle, and in doing so he opened the door to the age of Arthur.