A feast is underway when The Green Knight appears in King Arthur's hall:
"Nay, King Arthur," cried the knight in a great voice appropriate to his size. " I come here not to feed but rather to find the knight who has the courage to trade blows with me... he must strike me first, so powerfully as he will."
Therefore with great joy Gawaine did leap from his siege, take up the great ax, and with one blow strike off the head of the Green Knight, which went a-rolling the vast length of the great hall of Camelot, struck the far wall, and came rolling back unto the very feet of King Arthur. And the wondrous thing was that this head did roar with laughter throughout its journey to and fro! Then the green body rose, and taking up the green head, placed it upon the green neck, and mounted the green horse.
"Well struck, Sir Gawaine," cried the Green Knight. "And now that I have felt the strength of your arm, I shall test your moral mettle. One year from today, you must meet me at a place of my choosing, there to accept one blow from me, or else be damned as an arrant poltroon!" And guffawing he did prick his horse and gallop out of the castle.
From "Arthur Rex: A Legendary Novel"
by Thomas Berger
Delacourte Press/Seymour Lawrence, © Copyright 1978.
In some versions of the legend, the Green Knight is an ordinary man who is under an enchantment cast by Morgan Le Faye. In Berger's version, the Green Knight turns out to be the Lady of the Lake in disguise. But Gawaine does not discover this until he keeps his appointment with the Green Knight in the Green Chapel, after enjoying the freedoms of Liberty Castle.