Sir Gawain wanted to extract revenge on Lancelot, who had killed his brother Sir Gawain during a battle. Sir Gawain managed to convince King Arthur that Lancelot was a traitor. King Arthur put Morgan le Fay's son, Sir Mordred in charge of his kingdom and took his army to confront Lancelot.
Sir Mordred had eyes on becoming king and marrying Guinevere, so he announced that King Arthur had been killed in battle with Sir Lancelot. Hence Mordred became king. King Arthur heard this news and returned to England to regain his kingdom. Mordred's army had a pitched battle with Arthur's army at Dover. In the battle Sir Gawain was mortally wounded, but while dying asked the king to forgive Sir Lancelot. In the battle King Arthur and Mordred fought each other. Arthur and Mordred killed each other in single combat.
When news reached Lancelot of Arthur's death in battle, Lancelot exchanged his armour for the clothes of a monk. When he died, he was buried beside his friend Galehaut at Joyous Guard.
King Arthur and his Queen Guinevere and the Knights of the Round Table had come to an end.
Sir Lancelot, knight of the Round Table