Guinevere and Lancelot Caught!
Different resources attribute the plot to different people, but most often it is Mordred who sets a trap to catch Guinevere and Lancelot together. Once they are exposed in their betrayal, Arthur knows that the punishment for treason and adultery against the king is death.
Guinevere, the disgraced queen, was sentenced to death by being burned at the stake. Lancelot rescues her, accidentally killing Sir Gareth and Gaheris, and then they hid together in his castle, Joyous Gard, which Arthur then attacks. The siege was long and deadly. Eventually, they compromised. Lancelot returned to his homeland in France, and Guinevere returned to Camelot with Arthur. However, Arthur continued to pursue Lancelot.
While searching for Lancelot, Arthur left Mordred to rule in his place. Mordred quickly took this opportunity to declare himself king, and Guinevere his queen. Arthur rushed back to Britain to confront the uprising.
Guinevere escaped from Mordred's clutches and sought refuge in the Tower of London with some trusted knights to wait for Arthur's return. Mordred attacked the Tower, but did not succeed in recapturing the queen. Arthur rescued her, then continued his pursuit of his treacherous nephew.
Here's another version. And of course, another.
Guinevere, the disgraced queen, was sentenced to death by being burned at the stake. Lancelot rescues her, accidentally killing Sir Gareth and Gaheris, and then they hid together in his castle, Joyous Gard, which Arthur then attacks. The siege was long and deadly. Eventually, they compromised. Lancelot returned to his homeland in France, and Guinevere returned to Camelot with Arthur. However, Arthur continued to pursue Lancelot.
While searching for Lancelot, Arthur left Mordred to rule in his place. Mordred quickly took this opportunity to declare himself king, and Guinevere his queen. Arthur rushed back to Britain to confront the uprising.
Guinevere escaped from Mordred's clutches and sought refuge in the Tower of London with some trusted knights to wait for Arthur's return. Mordred attacked the Tower, but did not succeed in recapturing the queen. Arthur rescued her, then continued his pursuit of his treacherous nephew.
Here's another version. And of course, another.
Battle of Camlann
Arthur finally caught up to Mordred on the Salisbury Plain, and there they fought one last time, at the Battle of Camlann.
The night before the battle, Arthur had a dream where the ghost of Gawain told him not to fight Mordred, who was destined to deliver a mortal blow to him. He counseled that Arthur should wait a month and wait for the reinforcements, including Lancelot, to arrive. When Arthur awoke, he sent an offer of truce to Mordred, who agreed since he was also dreading the battle. The two agreed to meet and talk about terms. Before he left the camp, however, he warned his knights to be on the lookout for treachery, and attack if they saw a sword drawn.
During their meeting, one of the knights was bitten by a snake, and without hesitation he drew his sword and killed it. However, the soldiers on both sides of the battle saw the sword, and both sides attacked.
This was a long, bloody battle. At the end, only Arthur and his knight Sir Bedivere were left on their side. Mordred was the only one left on his side. Arthur rushed to kill Mordred, but accidentally dropped Excalibur and the enchanted scabbard. Arthur killed Mordred with a spear, but not before Mordred delivered a deadly blow to Arthur's head.
The night before the battle, Arthur had a dream where the ghost of Gawain told him not to fight Mordred, who was destined to deliver a mortal blow to him. He counseled that Arthur should wait a month and wait for the reinforcements, including Lancelot, to arrive. When Arthur awoke, he sent an offer of truce to Mordred, who agreed since he was also dreading the battle. The two agreed to meet and talk about terms. Before he left the camp, however, he warned his knights to be on the lookout for treachery, and attack if they saw a sword drawn.
During their meeting, one of the knights was bitten by a snake, and without hesitation he drew his sword and killed it. However, the soldiers on both sides of the battle saw the sword, and both sides attacked.
This was a long, bloody battle. At the end, only Arthur and his knight Sir Bedivere were left on their side. Mordred was the only one left on his side. Arthur rushed to kill Mordred, but accidentally dropped Excalibur and the enchanted scabbard. Arthur killed Mordred with a spear, but not before Mordred delivered a deadly blow to Arthur's head.
Arthur asked Sir Bedivere to return Excalibur to the Lady of the Lake. Sir Bedivere took his dying king and his sword to Avalon, where Arthur's body was placed on a boat with several maidens. The maidens take Arthur across the lake to Avalon, where he was to heal from his wounds. Sir Bedivere threw Excalibur into the lake, where it was caught by the Lady of the Lake, and returned to her once again.
The Once and Future King
Legend says that King Arthur is not in fact dead, but lying in wait in Avalon for a time when he is once again needed. In this sense, Avalon is not necessarily a real place, but a between-worlds region reserved for gods and spirits.
Other versions of this story are here
Other versions of this story are here