Who is sir mordred and why does he meet king arthur's ships with an army

Sir Modred or Mordred is usually said to be Arthur's  son by his half-sister Margause. He rebelled while Arthur was away fighting Lancelot in France and met him with an army when he returned. Arthur drove his lance through Modred, and Modred struck Arthur with his sword in the head. A hand comes out of the water to catch Arthur's sword. Then several ladies in a boat come to get Arthur. Some versions of the story say he went to Avalon and is still there but will return when Britain needs him most. A French chanson says Arthur and Huon of Bordeaux struggled for the rule of Avalon.