Geoffrey of Monmouth wrote Historia Regum Britanniae (The History of the Kings of Britain) sometime between 1135 and 1139. It is, as Wikipedia says, a “pseudohistorical account of British history”beginning with the settlement of Britain and continuing until the Anglo-Saxons assumed control of Britain around the 7th century.
It is significant in Arthurian literature for placing King Arthur in the context of British monarch history. According to Geoffrey, Vortigern conspired with the Saxons to usurp the throne. Aurelius, the rightful heir, wrested back power and was succeeded by his brother, Uther Pendragon, who was the father of King Arthur.
Geoffrey introduced many of the ideas which would become benchmarks for many Arthurian stories to come. He created characters such as Merlin, Uther Pendragon, and Guinevere. He also told stories of Arthur's conception at Tintagel, Excalibur, and the king's final days.
According to Geoffrey, Arthur assumed the throne at age 15 when his father dies. The new king fought a series of twelve battles against the Saxon barbarians, creating an empire including Ireland, Iceland, and the Orkney Islands.
Following twelve years of peace, he conquered Norway, Denmark, and Gaul. As he prepared to march on Rome, Arthur learned that his nephew Mordred, whom he left in charge of Britain, married Guinevere and seized the throne. Arthur returned to Britain and killed Mordred, but was left mortally wounded. He handed the crown to his kinsman Constantine and was taken to the isle of Avalon to be healed, but was never seen again.