Dunvalo Molmutius was born into a Britain shattered by civil war. His father was Cloten, the king of Cornwall, a petty realm in the southwest of the island. The great kingdom founded by Brutus, the Trojan exile who had given his name to Britain, had collapsed after the death of King Gorboduc and the murder of his two sons, Ferrex and Porrex. For years the island had been torn apart by the Civil War of the Five Kings, with Pinner ruling Loegria, Rudaucus ruling Cambria, Staterius ruling Albany, and two others carving up what remained. Dunvalo grew to manhood in Cornwall, and when his father died he inherited a small kingdom surrounded by larger, hungrier enemies.

The defining campaign of Dunvalo's life began when he marched against Pinner, the king of Loegria, and slew him in battle. This victory brought him to the attention of Rudaucus of Cambria and Staterius of Albany, who feared that the young Cornish king would upset the balance of power they had maintained through the civil war. The two kings allied their armies and marched into Dunvalo's lands, burning and pillaging as they came. Dunvalo met them with thirty thousand men, and for most of a day the battle hung in the balance, neither side gaining ground. Then Dunvalo devised a stratagem of such cunning that it became legend. He selected six hundred of his bravest warriors, including himself, and had them strip the armour from the enemy dead. Dressed as the fallen, they marched deep into the enemy lines until they stood before the two kings themselves. At Dunvalo's command they fell upon Rudaucus and Staterius and killed them both. The enemy army broke and fled, and Dunvalo pursued them from coast to coast, burning their castles and reducing their kingdoms to obedience.

With Britain united under his rule, Dunvalo set aside conquest and began the work of law. He caused a golden crown to be made and was crowned the first monarch of all Britain, restoring the island to the unity it had known under Brutus. Then he set about codifying the Molmutine Laws, a body of statutes that would make his name immortal. The laws granted sanctuary to any fugitive who reached a temple of the gods or a city, and extended that protection to the roads leading to those places and to the ploughs of the farmers who fed the realm. They allowed women to inherit the throne, a provision borrowed from the ancient Trojans who had settled Britain before Brutus. They protected the weak, the foreigner, and the bard, and they ended the robbery and murder that had flourished during the civil war. Geoffrey of Monmouth, the twelfth-century chronicler whose History of the Kings of Britain made the legend famous, wrote that the laws were so effective that in Dunvalo's day a man could travel from one end of the island to the other without fear of violence.

Dunvalo founded cities and raised monuments to the peace he had won. In Trinovantum, the city that would one day be called London, he built the Temple of Concord, a tribute to the harmony his laws had created. Some later traditions, including the fourteenth-century Short Metrical Chronicle, even claimed that Merlin himself served Dunvalo and built Stonehenge in his honour, though this confused chronology places the wizard centuries before his better-known association with King Arthur, the future king who would one day draw the sword from the stone and found the Round Table. Dunvalo also founded Malmesbury and the town that became known as the Vies, and he gave privileges to temples, to cities, and to the highways that connected them. He reigned in peace and prosperity for forty years, and when he died he was buried in the Temple of Concord on the White Mount, where Brutus himself had been laid to rest and where Bendigeid Vran, the giant king of Welsh legend whose severed head had once been kept as a guardian of the island, had also been interred.

Dunvalo Molmutius died not by the sword but in his sleep, an old man who had outlived the wars of his youth. His death, however, undid everything he had built. He left two sons, Belinus and Brennius, both of whom claimed the throne. Their rivalry ignited a new civil war that split Britain once more, proving that even the wisest laws and the strongest crown could not bind what ambition and pride would tear apart. Yet the Molmutine Laws endured. They were translated into Latin by the monk Gildas Priscus, then into English by King Alfred the Great, the Anglo-Saxon ruler who would one day fight off the Viking invasions of England, and they became part of the foundation of English common law. Shakespeare, the playwright whose works would define English literature, would later write in Cymbeline that a British king said: "Molmutius made our laws." The name of the lawgiver outlasted the kingdom he built.