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 . In medieval romances , one of the most prominent story arcs is Queen Guinevere 's love affair with her husband 's chief knight , Lancelot . Lancelot . 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 .