Morganus was a legendary king of the Britons as accounted by Geoffrey of Monmouth

Geoffrey of Monmouth

Geoffrey of Monmouth was a cleric and one of the major figures in the development of British historiography and the popularity of tales of King Arthur...

. He was the son of Maglaurus, Duke of Alba

Alba

Alba is the Scottish Gaelic name for Scotland. It is cognate to Alba in Irish and Nalbin in Manx, the two other Goidelic Insular Celtic languages, as well as similar words in the Brythonic Insular Celtic languages of Cornish and Welsh also meaning Scotland.- Etymology :The term first appears in...

ny, and Goneril, the daughter of King Leir

Leir of Britain

Leir is a legendary ancient king of the Britons, as recounted by Geoffrey of Monmouth. His story is told in a modified form by William Shakespeare in the play King Lear. In the drama, some names are identical to those of the legend Leir is a legendary ancient king of the Britons, as recounted by...

.

Morganus, grandson of Leir, despised the rule of his aunt Cordelia

Queen Cordelia

Queen Cordelia was a legendary Queen of the Britons, as recounted by Geoffrey of Monmouth. She was the youngest daughter of Leir and the second ruling queen of pre-Roman Britain. There is no independent historical evidence for her existence....

 in Britain

British Iron Age

The British Iron Age is a conventional name used in the archaeology of Great Britain, referring to the prehistoric and protohistoric phases of the Iron-Age culture of the main island and the smaller islands, typically excluding prehistoric Ireland, and which had an independent Iron Age culture of...

. With the help of his cousin Cunedagius

Cunedagius

Cunedagius was a legendary king of the Britons, as recounted by Geoffrey of Monmouth. He was the son of Henwinus, Duke of Cornwall, and Regan, the daughter of King Leir....

, Morganus took over the kingdom from Cordelia and ruled half of it. Following Cordelia's suicide, Morganus came to rule the region of Britain northeast of the Humber

Humber

The Humber is a large tidal estuary on the east coast of Northern England. It is formed at Trent Falls, Faxfleet, by the confluence of the tidal River Ouse and the tidal River Trent. From here to the North Sea, it forms part of the boundary between the East Riding of Yorkshire on the north bank...

.

Morganus was eldest male heir of Leir and, influenced by his peers, became discontent with ruling only half of Britain. He began a scorched earth march through Cornwall

Cornwall

Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...

 until reaching the army of Cunedagius. Cunedagius defeated Morganus and Morganus fled. Cunedagius followed him throughout Britain until cornering him in Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

. There Cunedagius killed Morganus and became king of all Britain. He named the place Margon in honour of his cousin.