King Arthur is a legendary British leader who, according to medieval histories and romances, led the defence of Britain against Saxon invaders in the late 5th and early 6th centuries AD, the sparse historical background of Arthur is gleaned from various sources, including the writings of Gildas, a 6th-century British monk, best known for his scathing religious polemic titled what?
King Arthur:  The sparse historical background of Arthur is gleaned from various sources, including the "Annales Cambriae", the "Historia Brittonum", and the writings of Gildas. Gildas: Gildas (Breton: "Gweltaz", c. 500–570) — also known as Gildas the Wise or Gildas Sapiens — was a 6th-century British monk best known for his scathing religious polemic "De Excidio et Conquestu Britanniae", which recounts the history of the Britons before and during the coming of the Saxons.
De Excidio et Conquestu Britanniae