Guinevere was a King.
Guinevere ( [ ˈɡwɪnᵻvɪər ] Gwenhwyfar Gwenivar ) , often written as Guenevere or Gwenevere , is , in Arthurian legend , the wife of King Arthur . She first appears as Guanhumara ( with many spelling variants in the manuscript tradition ) in Geoffrey of Monmouth 's pseudo-historical chronicle of British history , the Historia Regum Britanniae , written circa 1136 AD . Guinevere and Lancelot 's betrayal of Arthur preceded his eventual defeat at the Battle of Camlann by Mordred . Sir Lancelot du Lac ( meaning Lancelot of the lake , baptised as Galahad ) was one of the Knights of the Round Table in the Arthurian legend . He typically features as King Arthur 's greatest companion , the lord of Joyous Gard and the greatest swordsman and jouster of the age -- until his adultery with Queen Guinevere is discovered , causing a civil war exploited by Mordred which brings about the end of Arthur 's kingdom .