Iordains, Iurdains, Jordanus
Chamberlain of Gorlois, the Duke of Cornwall.
Merlin magically disguised either himself or Ulfius as Jordan to help sneak Uther and Ulfius into the castle of Tintagel so that Uther Pendragon could sleep with the Duke’s wife, Igerne. Jordan eventually became one of Arthur’s knights and participated in the war against the Saxons.
Are you looking for information about the river with the same name?
River Jordan | 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
Vulgate Merlin | 1220-1235
Le Morte Darthur | Sir Thomas Malory, 1469-1470