Gawain and the Green Knight.

One Christmas, a knight garbed all in green appears in King Arthur's Court and challenges any knight to hew off his head on the proviso that he be allowed to return the axe stroke afterwards. All of King Arthur's other knights being frightened, Sir Gawain takes up the challenge and hacks off the green knight's head. The green knight then gives Gawain a year to find his home and live a little before the cut is returned.