Geoffrey of Monmouth
Born in 1090, Geoffrey came of a Breton family and appears to have been schooled in the town's priory before continuing his studies and work at Oxford. Here he was entrusted with the task of translating a very old manuscript from the ancient British language into Latin. From this and other sources he completed his "History of the British Kings" which contains references to Cymbaline and Lear, the coming of Christianity, the departure of the Romans and the exploits of the legendary King Arthur.
Geoffrey tells that the British king Constantine had three sons: Constans, Aurelius Ambrosius, and Uther Pendragon. When Constantine died, his oldest son Constans emerged from a monastery to take the throne. In the absence of a willing bishop, the cons ul Vortigern crowned the boy, and then "began to deliberate with himself what course to take to obtain the crown, of which he had been before extremely ambitious."
Vortigern incited a group of drunk Picts to break into Constans' bedchamber and murder him. The dead king's brothers, Ambrosius and Uther, fled to Lesser Britain in fear of their lives, and Vortigern took the throne.
Meanwhile the Picts, outraged at Vortigern's treachery and his subsequent execution of the assassins, declared war on his kingdom, forcing Vortigern to invite Saxons into Britain to help him. He married Rowen, the daughter of the Saxon leader Hengist, but quickly found himself fighting his father-in-law and the Saxon warriors for his own lands.
The disinherited princes, Uther and Ambrosius, led armies from Lesser Britain, finally winning back their kingdom and burning Vortigern to death in the tower he had built to protect himself. After his second brother's death, lusty Uther fell passionately in love with the wife of Gorlois, Duke of Cornwall. He enlisted Merlin's aid to get him into Tintagel in the guise of the Duke.
"The king therefore stayed that night with Igerna, and had the full enjoyment of her, for she was deceived with the false disguise which he had put on, and the artful and amorous discourses wherewith he entertained her... Believing all that he said, she refused him nothing which he desired. The same night therefore she conceived of the most renowned Arthur."
Arthur succeeded his father to the throne and began his reign by subduing the Picts and bringing Ireland, Iceland, and Gothland under his rule, followed by Norway, Dacia, Aquitain, and Gaul.
Not content with this, Arthur decided to conquer Rome after being insulted by a tribute-seeking Roman ambassador. He defeated the Roman general Lucius Tiberius in Gaul, but "had news brought him that his nephew Modred, to whose care he had entrusted Britain, had by tyrannical and treasonable practices set the crown upon his own head; and that queen Guanhumara, in violation of her first marriage, had wickedly married him." Enraged, Arthur returned to Great Britain to punish the traitor.
Modred's Irish, Saxon, Pict, and Scottish troops met with Arthur's Bretons, and the ensuing battle destroyed both armies and both kings. "Even the renowned king Arthur himself was mortally wounded; and being carried thence to the isle of Avallon to be cured of his wounds, he gave up the crown of Britain to his kinsman Constantine ... in the five hundred and forty-second year of our Lord's incarnation."
[Giles, J.A., ed. Old English Chronicles. George Bell & Sons; London, 1908.]