Nennius is a prince of Britain at the time of Julius Caesar

Julius Caesar

Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

's invasions of Britain

Caesar's invasions of Britain

In his Gallic Wars, Julius Caesar invaded Britain twice, in 55 and 54 BC. The first invasion, made late in summer, was either intended as a full invasion or a reconnaissance-in-force expedition...

 in 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...

's legendary History of the Kings of Britain

Historia Regum Britanniae

The Historia Regum Britanniae is a pseudohistorical account of British history, written c. 1136 by Geoffrey of Monmouth. It chronicles the lives of the kings of the Britons in a chronological narrative spanning a time of two thousand years, beginning with the Trojans founding the British nation...

(ca. 1136). In Middle Welsh

Middle Welsh language

Middle Welsh is the label attached to the Welsh language of the 12th to 14th centuries, of which much more remains than for any earlier period. This form of Welsh developed from Old Welsh....

 versions of Geoffrey's Historia he is called Nynniaw.

He is the third son of Heli and brother of Lud

Lud son of Heli

Lud , according to Geoffrey of Monmouth's legendary History of the Kings of Britain and related medieval texts, was a king of Britain in pre-Roman times. He was the eldest son of Geoffrey's King Heli, and succeeded his father to the throne. He was succeeded, in turn, by his brother Cassibelanus...

 and Cassibelanus

Cassivellaunus

Cassivellaunus was an historical British chieftain who led the defence against Julius Caesar's second expedition to Britain in 54 BC. The first British person whose name is recorded, Cassivellaunus led an alliance of tribes against Roman forces, but eventually surrendered after his location was...

 (and according to Welsh sources, of Llefelys

Llefelys

Llefelys is a character in Welsh mythology appearing in the medieval Welsh tale Cyfranc Lludd a Llefelys. In the tale, Llefelys is king of France while his brother Lludd is king of Britain...

). He fights alongside Cassibelanus when Caesar invades. He and his nephew Androgeus

Mandubracius

Mandubracius or Mandubratius was a king of the Trinovantes of south-eastern Britain in the 1st century BC.-History:Mandubracius was the son of a Trinovantian king, named Imanuentius in some manuscripts of Julius Caesar's De Bello Gallico, who was overthrown and killed by the warlord Cassivellaunus...

, leading the troops of Trinovantum

Trinovantum

Trinovantum, in medieval British legend, is the name given to London in earliest times. According to Geoffrey of Monmouth's Historia Regum Britanniae it was founded by the exiled Trojan Brutus, who called it Troia Nova , which gradually corrupted to Trinovantum...

 (London

London

London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

) and Canterbury

Canterbury

Canterbury is a historic English cathedral city, which lies at the heart of the City of Canterbury, a district of Kent in South East England. It lies on the River Stour....

, encounter Caesar's own troop, and Nennius faces Caesar in single combat

Single combat

Single combat is a fight between two single warriors which takes place in the context of a battle between two armies, with the two often considered the champions of their respective sides...

. Caesar strikes Nennius a blow on the head, but his sword gets stuck in Nennius's shield. After they are separated in the melée, Nennius throws away his own sword and attacks the Romans with Caesar's sword, killing many, including the tribune Labienus

Quintus Laberius Durus

Quintus Laberius Durus was a Roman tribune who died during Julius Caesar's second expedition to Britain. Caesar describes how soon after landing in Kent, the Romans were attacked whilst building a camp by the native Britons. Before re-inforcements could arrive, Laberius was killed...

. Fifteen days after the battle Nennius dies of his head wound, and is buried at London, near the North Gate. Caesar's sword, named Crocea Mors

Crocea Mors

Crocea Mors was the name given to Julius Caesar's sword, according to the legends presented by Geoffrey of Monmouth. In Middle Welsh versions, it is called Angau Coch or Agheu Glas ....

("Yellow Death"), is buried with him.