Brother of the Cornish king Caradoc. Dionotus assumed the kingship of Cornwall upon his brother’s death.
Maximus made him the ruler of Britain in his place when he went to conquer Gaul. Dionotus became an ally of Conan Meriadoc – ruler of Brittany – when Conan married his daughter Ursula.
Dionotus helped Conan import thousands of British women to ‘Other Britain’ to help spread British influence throughout Gaul.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138