If Nennius established Arthur as a British hero, Geoffrey of Monmouth brought him to life. Born around 1090, possibly in Wales, and educated in Paris and Oxford, Geoffrey was ensconced as a bishop in Britain in the mid-1100s when he wrote, in Latin, perhaps the most influential book ever about Arthur. Like Nennius, Geoffrey had a political agenda: to show the superiority of the Celtic-speaking Britons, and by extension, the Welsh, who spoke the same language. “There were a lot of criticisms of the Welsh as being savages, barbarians,” says Morris. “Geoffrey invents a noble history for them, going back a hundred kings before Arthur.” A cipher in Nennius’ history, Arthur now became a Celtic-speaking warrior-king within a richly imagined narrative.

Source: Was King Arthur a Real Person? | History | Smithsonian Magazine