Geoffrey of Monmouth, << JEHF ree of MON muhth >> (1100?-1155?), was a Welsh historian who wrote the first narrative of the entire life of King Arthur, a legendary ruler of Britain. Geoffrey tells of Arthur in History of the Kings of Britain (about 1136). The book is a mythical story of Britain’s rulers from its first king, Brut (whom Geoffrey calls Brutus), through the rulers of the A.D. 600’s. Geoffrey claimed to have based his work on a chronicle. But most of the tale is probably his own invention. See also Arthur, King.