A group of twelve knights who Guerin of Chartres brought to pay homage to Arthur at his coronation feast in Caerleon. They also joined Arthur for the Roman War.
Geoffrey’s allusion to the Twelve Peers seems to be an attempt to compare the greatness of Arthur’s court to that of Charlemagne in continental romance.
Sources
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
Roman de Brut | Wace, c. 1155