Sater, Staterius
According to Geoffrey of Monmouth, the ruler of Demetia or Albany, serving Arthur.
He and Rudaucus, King of Cambria, joined forces against Molmutius (Dunwallo Molmutius), but they were defeated and killed in battle.
Geoffrey apparently took his name from Welsh mythological genealogies.
Sources
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
Roman de Brut | Wace, c. 1155
Brut | Layamon, late 12th century to mid-13th century