A British tribal leader or king of the first century BC, the ruler of the tribe of the Catuvellauni whose territory lay along the banks of the river Thames and who led the British resistance against the incursions of the Romans under Julius Caesar in 55 and 54 BC.
Prior to Caesar's invasions Cassivellaunus was busy spreading his authority over southern Britain; he had invaded the neighbouring territory of the Trinovantes (who essentially occupied what is now Essex), killed their king Imanuentius whose son Mandubratius promptly fled and sought refuge under the protection of Julius Caesar. The pretext of restoring Mandubratius was the excuse that lay behind both of Caesar's invasions.
One theory is that the name 'Cassivellaunus' should be translated as 'Vellaunus of the Cassi' (since, whereas Julius Caesar does not mention a tribe by the name of the Catuvellauni he does mention a tribe by the name of the Cassi) with Vellanus either being a personal name or a word meaning ruler. It could therefore be the case that the tribal name Catuvellauni is itself derived from the same source and means 'warband of Vellaunus' and indicates the possibility that this Vellaunus was a Brythonic warlord carving out his own kingdom in pre-Roman Britain and that the Catuvellauni was a tribal federation of his own making.
In any event Julius Caesar tells us in his De Bello Gallico when speaking if the opposition to his advance that the British tribes had "... by common consent ... entrusted the supreme command and conduct of the campaign to Cassivellaunus". Julius Caesar defeated Cassivellaunus' forces in a battle fought on the banks of the Thames and then chased him back to his stronghold. The Romans then stormed this stronghold 1 forcing Cassivellaunus to flee and sue for peace, which was granted by Caesar in return for providing the usual hostages and the payment of an annual tribute.
Cassivellaunus died in around 30 BC, the name of his successor is not known. 2 In Welsh tradition, his name is preserved as Caswallon or sometimes Caswallawn and appears in the odd triad here and there.
1 The location of which is not known, but informed opinion suggests Wheathamstead in Hertfordshire.
2 Probably on of his sons who died in around 20 BC bequeathing the Catuvellaunian kingdom to his son Tasciovanus.