Sir Agravin and Sir Mordred told King Arthur that Launcelot loved the Queen
Please lets put an end to this
I´ll never forgive you for the deth of my brothers
The twelve nights went to the Queens room where Launcelot was. They told him to come out but he didn´t. Then he came out without armour, only his sword. They fight and he run away.
King Arthur order Sir Gaheris and Gareth to kill the Queen in the fire. But when the torch was lit, suddenly Sir Launcelot came, and saved the Queen. Without knowing it, Launcelot killed Gareth and Gaheris.
Everyone at the Kingdom specially Sir Gwain begged the King for revenge of Launcelot. So they all went to his castle. They had a big fight. But they made a deal, Launcelot will give the Queen and he will leave to Armorica until they may need him.
After a while Sir Gwain and Arthur decided to fight Launcelot so they went into Armorica, leaving Mordred to ruled Britain while he was away. Sir Gwain fight Launcelot without victory. Meanwhile Mordred won enough Knights to convinced them that King Arthur had died and to chose him as their King. He also tried to convice Guinevere to merry him, but she went to hide and to send the message to Arthur. They returned and Mordred was already waiting for them.
They had a big battle until Sir Mordrer leave. King Arthur saw Gwain lying mortally wounded, so he said good bye and before he died he wrote a letter to Launcelot saying that he forgave him and that King Arthur needed him.