Sir Gawain The Green Knight -
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The story starts with the invasion of King Arthur's court at New Year by the terrifying and mysterious figure of the Green Knight who lays down a challenge to the assembled court, the challenge being that a volunteer must strike off the Green Knight's head with his axe but must present himself for a return blow a year hence. Only Gawain has the courage to meet the challenge and he strikes off the Green Knight's head with a single blow, only to have the Green Knight pick up his head, mount his horse and ride out of the court.
http://www.whitedragon.org.uk/articles/gawain.htm
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I even saw a fairly good movie adaptation a couple of years ago. I wish I could remember its name. It didn't look like something one of the big movie houses would had made, but then it could have been an older flick. I ought to try looking it up... 
