who was this king?
He was 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.
King Arthur
King Arthur (, , ) was 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. 
who wrote the first book about king arthur The familiar literary persona of Arthur began with Geoffrey of Monmouth's pseudo-historical "Historia Regum Britanniae"
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King Arthur [SEP] Introduction
King Arthur (, , ) was 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. The details of Arthur's story are mainly composed of folklore and literary invention, and modern historians generally agree that he is unhistorical. The sparse historical background of Arthur is gleaned from various sources, including the "Annales Cambriae", the "Historia Brittonum", and the writings of Gildas. Arthur's name also occurs in early poetic sources such as "Y Gododdin". Arthur is a central figure in the legends making up the Matter of Britain.