Geoffrey of Monmouth 
Historia regum Britanniae 
Geoffrey’s British History

Historia regum Britanniae, originally called De gestis Britonum (On the Deeds of the Britons), is a pseudohistorical account of British history, written around 1136 by Geoffrey of Monmouth. It chronicles the lives of the kings of the Britons over the course of two thousand years, beginning with the Trojans founding the British nation and continuing until the Anglo-Saxons assumed control of much of Britain around the 7th century.
The Wanderings of Brutus
Guendoloena, Bladud, King Leir and His Daughters, Dunwallo Molmutius
Belinus and Brennius, Elidure, Heli, Lud
Cassibellaun and the Invasions of Caesar; Kymbelinus; Arviragus and the Invasion of Claudius; King Lucius Becomes a Christian
Roman Britain From Severus to Maximian
The Romans Leave; Vortigern; the Saxons Arrive, Led by Hengist and Horsa; and Introducing Merlin
The Prophesies of Merlin
Aurelius Ambrosius, Merlin Builds Stonehenge, Uther Pendragon, the Birth of Arthur
The Victories and Ceremony of Arthur’s Reign; the Romans Demand Tribute
Arthur’s War Against the Emperor Lucius Tiberius in Gaul
Arthur’s Last Battle, Against Modred; the Saxons Return; Civil War; Augustine Comes to Convert the Saxons
Decline of the Britons Under Cadwan, Cadwalla and Cadwallader

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Geoffrey of Monmouth (c. 1095–c. 1155) was a cleric from Monmouth, Wales, and one of the major figures in the development of British historiography and the popularity of tales of King Arthur. He is best known for his chronicle The History of the Kings of Britain (Latin: De gestis Britonum or Historia Regum Britanniae) which was widely popular in its day, being translated into other languages from its original Latin.