Arthur and the Lost Kingdoms

The Holy Grail, the kingdom of Camelot, The Knights of the Round Table and the magical sword Excalibur are all key ingredients of the legends surrounding King Arthur. But who was he really, where did he come from, and how much of what we read about him in stories that date back to the Dark Ages is true? So far historians have failed to show that King Arthur really existed at all, for a good reason...