The king of Britain prior to Constantine (Arthur’s grandfather). He had previously been a general under the British king/Roman emperor Maximus, and had proved his prowess by driving the Picts and Huns off the island. As a king, however, he was tyrannical, and was assassinated by his own noblemen. Afterwards, the kingdom fell into ruin until the arrival of Constantine from Brittany.
Giovanni Boccaccio, the Italian author of The Fates of Illustrious Men, thought that Constantine obtained the kingdom from Gratian.
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
Roman de Brut | Wace, c. 1155
De Casibus Virorum Illustrium | Giovanni Boccaccio, 1355-1362