Why does the legend of King Arthur hold such a powerful grip over us?

You're right. The legend of King Arthur is one of the most powerful, and fascinating, stories ever — and for many, many reasons. It has all the elements of a great story: drama, adventure, romance, conflict and betrayal, interesting characters — just to name a few — plus the magical elements (though the King Arthur legend would still be a great story without the magic). Also adding to the fascination is that the legend may be based in historical fact; some scholars believe that a historical figure like King Arthur actually existed in Britain in the late 5th or early 6th century.
Over the centuries, many authors and filmmakers have given us their take on the legend — from novels like T.H. White's Once and Future King and Thomas Malory's Le Morte d'Arthur to films like Excalibur and Monty Python and the Holy Grail. As the legend of King Arthur continues to thrive as a cornerstone of British mythology, we'll see more versions that expand upon and add to the evolution of the legend.