What led to the downfall of King Arthur's kingdom?

In the King Arthur legends, the affair between Lancelot and Queen Guinevere caused a major rift in loyalty between the knights of the round table. There was lots of conflict and a siege in which Lancelot finally gave up Guinevere to King Arthur. Lots of the central knights were killed either accidentally or intentionally during this bout when trying to capture Lancelot and when Lancelot rescues Guinevere from execution. Mordred and Morgana Le Fay/ Morrigan Le Fay, were basically brewing up this whole conspiracy so that they could overthrow the king. There is a grand final battle, and Arthur loses. He is sent off to Avalon, the land of the elves, and is rumored to come back when the people of Britain need him most.