Marran, Mauron
A kinsman of Arthur who was serving as a soldier of Baldulph, the Saxon.
Apparently deciding that blood was thicker than water, Mauron betrayed his commander and warned Arthur of Baldulph’s plan to ambush him. Arthur was able to send Cador to slaughter Baldulph’s forces.
Arthur later appointed him the Earl of Worcester or Winchester.
See also
Earl of Winchester | The Legend of King Arthur
Sources
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
Roman de Brut | Wace, c. 1155
Brut | Layamon, late 12th century to mid-13th century