Lancelot and Guinevere
Guinevere first appears in written stories about 1325 AD in the Welsh story Culhwch ac Olwen, as Arthur's queen. In later medieval stories, she's the daughter of King Leodegrance, and she marries Arthur when they are both still very young. Arthur needs sons to inherit his throne, but Guinevere does not have any children. This makes her and Arthur very unhappy.
Finally Arthur's nephews Mordred and Agravain tell Arthur about Lancelot and Guinevere, and Lancelot flees for his life. Arthur captures Guinevere and orders her to burn at the stake (compare the older Parthian story of Vis and Ramin). At the last minute, Lancelot rides in on his horse and saves Guinevere, but he kills the knights Gaheris and Gareth.
King Arthur has to go to France to fight Lancelot and the Roman Emperor, and he leaves Mordred in charge. But Mordred makes himself king, and makes Guinevere marry him. Arthur comes back to save his kingdom and Guinevere. After the battle between Arthur and Mordred, Guinevere and Lancelot meet one last time, and then Guinevere enters a convent and stays there for the rest of her life.